Jumo 222: what's the truth?

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In all fairness the large number of Jumo 222 manufactured may be do to accounting practices. For example the V-1710-A1 Build 1 and Build 2 are considered the same engine by Allison yet Build 2 had every part replaced! Is it really the same engine if you do that? Junkers may have listed this as a new engine instead of a rebuilt one. Furthermore the total could include spare equivalents. For example, if one includes spare equivalents for the Chrysler I-2220 the number built goes from 6 to at least 50.
 
I am not qualified to have my own opinion as I haven't seen all primary sources which were abiable to Budrass but he expressed that the Jumo-222A1 was certified to be ok for mass production early 1942 based on a 30min rating of 2000hp on B4 fuel after passing bench flight tests. They even had a new factory tooled up in Vienna. It would also be ok for the proposed Ju288A to reach predicted performance.
However, already late in 41 Milch called for a change from 3men crew to 4 men crew, larger MTOW and more remote def guns requiring a larger wing and a 2500 hp class engine.
 
Well it sounds like if the Ju288A and the original version of the Jumo 222 were stuck with then they would be in production by late 1942-early 1943. Its the increased spec requirements, done on purpose, that killed both projects; Milch apparently made this his #1 project when he took over after Udet's suicide to dismantle the system of power that was in place to ensure his authority over the aviation sector, which Koppenberg had wrestled into his control with the Ju88 program; it seems Koppenberg was aiming for total monopoly and was trying to ensure with the Ju288 that he would kill his competitors and become the defacto minster of aviation himself, because he would control the only viable aviation firm at that point. So both the Jumo 222 and Ju 288 were not killed by technical issues, but rather administrative sabotage for political reasons.
 
I dopn't think Ostmark was ever actually tooled up to produce Jumo 222s. Or at least it was converted to DB 603 before the proses was complete. Regardless of whether or not the Jumo 222 was considered production ready, the sheer size and complexity of the crankcase casting could have been grounds for cancellation. Most certainly it would have made the unmitigated disaster that Ostmark was even worse.
 
I
dopn't think Ostmark was ever actually tooled up to produce Jumo 222s. Or at least it was converted to DB 603 before the proses was complete.
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Nope,

Ostmark was tooled up for the Jumo 222 inclusive all sub companys, which delivers fuel injections, supercharger and so on.

This was one of the reasons that the change of Ostmark to retool to the DB 603 cost two years.
 
How was it any different than the Napier Sabre? That engine was just fine after the initial hiccups were worked out.
 
This seems a little too simplistic and pat. It also seems to assume that the both the Ju288 and Jumo222 were without any problems that could not be easily solved. Politics may very well have been involved but even in politics you need a germ of truth or an excuse.

1937-start of work on the Jumo 222 engine
1938-planning work on the EF-73 project which becomes the basis for the Bomber B specification issued in July 1939.

Specification calls for a 600kph speed ( 373mph) and a bomb load of 4000kg (8820lbs) to be carried to any part of Great Britain form bases in France or Norway ( at least according to one source). Two 2500hp class engines are specified. Please compare specified performance to what the Douglas A-26 was able to achieve in 1943-44.

May 1939 sees H. Hertel leave Heinkel and join Junkers to oversee the EF 73 project. The Jumo 222 ran for the first time on a test stand the moth before. The EF 73 has an extensively glazed pressurized cockpit and remote control gun barbettes. The RLM requests (among other things) a larger wing and a change in the gun barbettes.
By the end of May 1940 a full mock up of the fuselage has been inspected and orders are given for 3 prototypes ( Junkers had already begun cutting metal) fuel tanks in wings and fuselage are to house 5360 liters ( 1416 US gallons again compare to A-26) of fuel.

Now 1940 is were things get a little strange, The Jumo 222 makes it's first flight Nov 3 1940 in the nose of a Ju 52. JU 288 V1 flies for the first time Nov 29 1940 using BMW 801 engines. The V2, V3 and V4 follow from March until May of 1941, The Jumo 222 is flown supposed to have passed it's type test in May of 1941? The JU 288 V5 with Jumo 222s flies Oct 1941 but ignition troubles keep engines from developing full power.

It is at this point that politics may or may not rear it's head. Plans had been made to produce 380 planes a month. The RLM now changes to the 4 man cockpit and other changes, including a yet bigger wing, which is tested on V6 with Jumo 222 engines (Jan 1942) and is lost due to engine fire. The V7 goes goes back to BMW 801s and seriously under powered it too is lost due to an engine fire. The V8 is flown with Jumo 222 engines as is the V9 in May 1942 (prototype for the B series). March of 1942 had seen orders for the production of the Ju 288 go ahead but by November the the numbers were cut to 35 aircraft due to the problems with the Jumo 222 engine, the C and D versions not performing satisfactorily and the engine project is back in development. The Ju 288 V10 flies in Jan 1943 with a pair of BMW 801TJ ( turbo-charged) engines. Earlier flights were made by the V11 and V13 with DB 606 engines in July and Sept 1942. The V12 and V 14 were the last Ju 288s powered by Jumo 222 engines.

I would note that they were using 1600hp BMW 801 engines for test flying instead of not only the later 2500hp Jumo 222 models but the earlier 2000hp models that some people seem to think were production ready ( or even built in a small production batch/s)

According to this account 6 Ju 288 prototypes flew with Ju 222 engines. It could be in error.

Again try compare what they were trying for to the A-26. More bombs, more fuel, a pressure cabin and more speed ( maybe achievable at the higher altitude ?) and they started 2 years earlier. By the time they got done (and saddled the plane with DB 610 engines) they had a plane that weighed more empty than the A-26 was supposed to weigh ready for take-off ( A-26 cheats as the difference between normal and Max weight is an easy 8,000lbs or so).

The Specification was too big a leap for the technology of the time which lead to an extended development time. The Initial design (EF 73) had a wing span smaller than an "A-20" but wanted 4 times the bomb load and more range ( fuel ) and used more powerful engines. Were ALL of the RLM requests political meddling or an attempt to bring the "design" somewhat closer to service requirements?
 

The Jumo 222 was ordered uprated in October 1941, though it had already undergone one displacement upgrade between 1939-41 (not sure when exactly) that bumped it up to 2500hp. So the initial change happened somewhere in 1941 prior to October, but the second increase which netted the C/D series of 3000hp engines (first run in summer 1942). Yet after this engine failed they went back to the original displacement in 1943 and fixed the problems, resulting in the E/F series.

So there were problems between 1939-1941, but it was the first displacement upgrade that started the development creep that prevented it from being ready in 1942. Wikipedia has unsourced claims that Junkers ordered the displacement increase, but I think that was the start of RLM interference.
Milch was starting his political counterattack on Udet and was issuing orders in October 1941, which ultimately resulted in Udet's suicide, so its likely that he was ordering unnecessary upgrades in the weight of the airframe around this time (such as adding a fourth crew member and more defensive armament among other things), while then demanding upratings in engine performance, something he knew would set back engine development.

The article below mentions the year delay after the April 1941 test, which would have put full scale production back until April-May 1942, but by then the order to move up to 2500hp was ordered and new problems set as the old ones were being solved. So the 2000hp version would have been ready in 1942, but after the March date Dave mentioned.
 

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The A-20 is a technically greatly inferior airplane design to the Ju-288 and the A-26 is not yet on par with it. I don´t know why it should be compared at all. After all, the amount of R&D which went into the combined JUMO-222&Ju-288 project was similar to that of the V-2 missile project. It had much more R&D support than the A-26. More effecient fuselage space usage (just think of bombbay and fuel tanks), more optimised structural framing, better ergonomically laid out cockpit, better preperation for section mass production and last but not least more advanced aerodynamics with much more time spent in the windtunnel for detail solutions.

There were not many Jumo-222A1 produced and the A1V-11 to A1V-16 were worked very hard in the trials, logging several hundred hours of flight. A document dated to 18th of oct. 1943 lists the Ju-288V05 with it´s n´th set of engines stationed at JUMO and given care and maintenance. Most of the JUMO-222 which were produced were of -A3 subvariants and late -E2/F2 subvariants for which after start of the Jägernotprogramm all suitable carriers were canceled from production list while single engined projects with JUMO-222 were not yet developed to the degree to carry this engine.

To blame it a technical fault is incorrect and Budraß is entirely correct in pointing out that the problem lies in the decision making process. One thing which is often overlooked is that in 1941 nobody expected a long continuation of the supposed to be short remainder of the war. The Ju-288 should represent the next generation bomber, not just a mildly improved version of existing ones. And while allied engeneerers succeeded in developing good to very good solutions and bringing them to front service in record breaking times, german ones didn´t succeeded in bringing them to front service because they tried to over-improve , over-engeneer and over-develop them, in part explainable by the formidable technical challanges in front of them. But not entirely, part of it was a prevalent low tolerance to compromises. Remember, german technicians in 1941/2 experimented with strange futuristic projects, like jet and rocket propelled aircraft and bombs, guided stand off weapons, long range missiles, turbine driven submarines.

The JUMO-222V05 driven by two JUMO-222A1V engines and a span of 60.3ft was the way to go. It offered the performance required with the test flights starting in september 1941. The plane was started beeing assembled by oct. 1940. It remained to be the only test bed according to the original -A1 specification with 3 men crew and small wing. The following change in general size meant that a whole pletora of other changes had to be done (gears, wings, cockpit, structure) so that the construction of the next prototype, the -V6 was not started before july 1941. That´s 3/4 of a year delay. This means that only the Ju-288A1 had a realistic chance of production (preproduction models in 1942 mass production scheduled for 1943 with 1st frontline service in 1943).
 
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We appear to have a timing disconnect.
The Jumo 222 was started in 1937 as a 2000hp engine.
The Bomber "B" requirement was issued in 1939 ( two years later) for 2500hp class engines.
Jumo 222 is already behind the curve. Now many engines did pick up 25% in power from early models to later ones (or more) but trying to pick up 25% in power while still in development and without the use of improved fuels is going to be difficult.

AS for "The A-20 is a technically greatly inferior airplane design to the Ju-288 and the A-26 is not yet on par with it. I don´t know why it should be compared at all. After all, the amount of R&D which went into the combined JUMO-222&Ju-288 project was similar to that of the V-2 missile project. It had much more R&D support than the A-26."
I think you are missing the point. The A-20 was a "technically greatly inferior airplane design" BUT it was in production in 1940 and I only used it to serve as a bench mark for what the Germans were trying to do. More speed, quadruple the bomb load and longer range in a similar size (but not weight ) aircraft. It certainly does require a more advanced aircraft. And here we hit part of the problem, perhaps it was too advanced or too ambitious. A 2000kg payload at the speeds and ranges requested would have been pretty challenging, trying for 4000 kg bomb load? The A-26 used a similar sized wing to the 288B and used a Laminar flow airfoil and double slotted flaps ( I know it is not true laminar flow but it was about the best air foil around at the time of it's design) and had 2000hp engines, a smaller bomb load and less fuel. You are going to need a really advanced aircraft to beat it for speed, range (fuel capacity) and larger bomb load while using the SAME power engines, yet the RLM is blamed for wanting 2500hp engines to meet the performance specifications.
Using up the resources of the V-2 program for a twin engine bomber may NOT be a bragging point. The Ju 288 may very well have been more advanced than the A-26 and used up a lot more R&D but you can't have it both ways. A long, costly and complicated development program IS much more subject to political interference.

I agree with " german ones didn´t succeeded in bringing them to front service because they tried to over-improve , over-engeneer and over-develop them, in part explainable by the formidable technical challanges in front of them. But not entirely, part of it was a prevalent low tolerance to compromises. Remember, german technicians in 1941/2 experimented with strange futuristic projects, like jet and rocket propelled aircraft and bombs, guided stand off weapons, long range missiles, turbine driven submarines."

But to put a fair amount of the blame on "politics" when you are trying to produce such long lead time weapons is not quite fair.

"The JUMO-222V05 driven by two JUMO-222A1V engines and a span of 60.3ft was the way to go."

It might have been if they left a few things out. You have a about 58lb per sq ft of wing area with a 'Normal" loaded weight of 33850lbs. The Whole bomber "B" program was way too much of trying to be "all things to all men". A less ambitious plane might have succeeded with the original Jumo 222 engines.
 
I am curious because I have the copies of the TA Bomber B requirements right in front of me but I am sorry, there is no word of a 2,500hp engine. The specifications call for a top speed of 600km/h, a max range of 3600km with 2ts payload and max. permissable 6ts short range overload payload. No engine is specified.

This is often mixed up with Junkers initiative proposal EF-74. This preliminary proposal based upon the Ju-288 layout, but was driven by 2 x JUMO-223 Aero Diesel engines with a supposed to be 2,500hp short term 5 min power. These engines didn´t existed yet in 1939. The JUMO-223 is not comparable with the JUMO-222. The entrance of the Junkers EF-74 and EF-73 in effect triggered the decision of the RLM to summon the bomber-B competition.

The JUMO-222A1 was perfectly capable of higher short term power. The 2000HP rating is 30 minutes rating. It wasn´t ever cleared for the 2480HP -5min take off rating (they just upped the requirements so that the 30 min. rating matched this performance). That´s still a bit short of the 2000hp planned max sustainable but quite close. Just to remember, the 1,750 HP BMW-801 required C3 fuel, the JUMO-222 was fine with low grade B4 fuels. A shift to C3-grade fuel would have allowed short time emergancy powers in excess of 2,200HP. It was tested for this.

The Ju88 was also in production in 1940, whats the point? Both are a generation older than the Ju-288. And a generation between 1938 and 1942 means quite a lot in terms of technical capabilities.


Fuselage cross section? Wetted area? iengine nacelle drag? Cooling drag? The JU-288 airframe aerodynamic studies alone took the Junkers company 60 mio. RM, about as much as 1,200 A4 missiles would have costed. They found a perfect integration between engine and nacelle which allowed for less wetted drag compared to older JUMO-211 engine installations in the Ju88, despite having a much more powerful engine. Just looking at the cockpit, the JU-288A1´s is just 97cm wide and defines the width of the hull. It´s a fighter style one person cockpit while that of the A26 offers 156cm width. The Junkers design was laid out for minimum frontal and wetted drag and high cruise speed at the expanse of rather high take off and stall speed and less stellar high altitude performance, a normal trade off by the standarts of the mid and late 40´s, but new in the late 30´s. They attempted to mediate by high lift aids and excess power (the proposed A/C was 15% heavier than the Ju88 but had 35% more power). It may, however, as a very fine tuned product have been less tolarable to structural weight changes in general.

A long, costly and complicated development program IS much more subject to political interference.
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True. but I really suggest You consult Prof. Budraß work. Just look at the critical decision making timeline between the suicide of Ernst Udet in nov. 1941 and Milch´s decision to change engine and airframe requirements in order to stop production in dec, the same year. The program was costly because of Milch´s repeated changing requirements which made sure it will never proceed beyond prototype stage. To many minutes noted how these changes compromised the performance envelope and had to be adressed by time consuming changes again... Just look how the project grew in size:

EF-73:
MTOW: 14.6t
wing area: 48 m^2

Ju-288A1 (mid 1940):
MTOW: 15.6t
wing area: 54.0m^2

Ju-288A2 (dec. 1941):
MTOW: 17.3t
wing area: 60m^2


Agreed but in defense of Junkers, the general trend was to go for higher wingload as it represents a tradeoff for better flight performance. The step was already taken by Messerschmidt with his Bf-109 earlier, the Focke-Wulf and Heinkel companies had very good experiences with this approach from various A/C projects, too but the Junkers company was rather late in applying this step. The approach was endorsed by Udet. Milch just rejected it.
 
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DonL said:
I

Nope,

Ostmark was tooled up for the Jumo 222 inclusive all sub companys, which delivers fuel injections, supercharger and so on.

This was one of the reasons that the change of Ostmark to retool to the DB 603 cost two years.
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Construction of the main building did not begin until late July 1941. It was completed around March 1942. It took even longer for the power plant, test cells and warehouses to be completed. The decision to switch over to the DB 603 was made in December 1941. First DB 603 deliveries were in May 1943. The earliest projected date for Jumo deliveries I have seen is April 1943.

I do not see how retooling cost two years.
 
Sorry, but I do not have copy of the specification Just books that apparently have it wrong. I stand corrected. But is speaks to the confusion about the Ju 288 and the Jumo 222 engine.

The Ju88 was also in production in 1940, whats the point? Both are a generation older than the Ju-288. And a generation between 1938 and 1942 means quite a lot in terms of technical capabilities.
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Yes it does and the point was simply how far the Design team was trying to go in one in one jump. I am making no claims as to which was better ( the Ju 88 or the A-20) just picking out a convenient benchmark.

If you wish go from the JU-88A-1. Wing about the same size and span (or with a few %) The 288 was to go from 1680 liters of fuel in the wing to 3560 liters and while the older plane could put another 1880 liters in the bomb bay (totally blocking it) the 288 was supposed to hold another 1800 liters in the fuselage without blocking the bomb bay (unless this book is in error and this tankage does block the bomb bay), this is while upping the bomb load by at least 50% (or 100%?) , fitting heavier defensive armament, the Pressure cabin and the heavier engines. And requiring about a 33% increase in speed. AND the first couple of 288 prototypes had dive brakes.

It is a wonder they didn't require it to tell jokes and wash windows
 
The Ju 288 carried fuel tanks above the bomb bay in special tanks, making sure to use every inch of space in the fuselage for bombs or fuel. It could also carry two external drop tanks if needed.
 
Why I don't think that fuel was carried in 'special tanks', seem like the whole plane was designed around it's bomb bay and fuel tanks. Was there any fuel int he wings? Plus, the pilot and co-pilot of the A-26were seated tightly side to side, while the crew compartment of the Ju-288 allows for a comfortable quarters - ie. not very much narrower than of A-26 (if any?), and the side blisters add to the drag, too.

added: there is also the lower blister, and top of the crew compartment is well above fuselage, so IMO there is no advantage in streamlining vs. A-26 front crew compartment.

 
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Shortround6 said:
Sorry, but I do not have copy of the specification Just books that apparently have it wrong. I stand corrected. But is speaks to the confusion about the Ju 288 and the Jumo 222 engine.
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Definitely. One has to remain beeing sceptical with so many things in secondary accounts.
Junkers was the single sole company which had both- aircraft and aeroengine R&D and manufacturing.




They thought that they could do this by adopting new structural layout. The load was shared between frames and the counterstressed skin as opposed to all by frames in the Ju-88. As mentioned above, the tanks (blue) were carried by the main structural member (see below, red) to which the hardpoints for the bombs were attached below. One unit, two functions and spacially optimised:



This way also offered more space for the internal bomb bay, which was dimensioned in a way to just take the two of the heaviest then possible bombs, the SC2500. In addition to the four 450ltr fuselage fuel tanks, there were two inner wing tanks with 720ltr each and four outer wing tanks with 660 and 400ltr each, respectively. Additional fuel could be carried in the bomb bay (4 x 450ltr tanks) and as drop tanks at the outer wing station (2 x 900ltr drop tanks), allowing for a max fuel load of 7,160ltr. Depending on choosen bombload, the fuel tankage varied.

there is also the lower blister, and top of the crew compartment is well above fuselage, so IMO there is no advantage in streamlining vs. A-26 front crew compartment
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The blisters were rather small and teardrop shaped. The fuselage of the A-26 was (similar to the Ju-288.. likely because of space optimisation) roughly squarish with rounded corners developing 85% -288- and 83% -A26- cross sectional coverage, respectively).
The A26 fuselage was 5 ft and 2.04inches wide and 5ft, 10inches high. This translates to a width of 1.57m and a height of 1.78m.
The Ju-288 fueslage cross section was 97cm wide and 1.40m high.
Cross sectional area of the Ju-288 fuselage (corrected for coverage) was 1.16m^2 while that of the A-26 was 2.32m^2, roughly twice as much frontal exposure.
At the position of the blisters, the width is 1.28m max and the height is 1.68m max. Still smaller than the A-26.

for the JUMO-222 I suggest the following datas aviable here:

http://www.enginehistory.org/German/Jumo222.shtml

There are still some mistakes in them. 2,500HP Powerrating 1.6 ata f.e. was never clerared for the A-1. The A-2 and A3 were run in 1942 with 1.78ata and 3000HP take off power. The Ju-288AV-05 refitted with JUMO-222 A3/B3 made 660km/h recorded top speed.
 
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I thank you both for the pictures and information. It looks like the Junkers engineers did a great job of packaging.

That doesn't mean that the Ju 288 wasn't a leap too far. While packaging explains how they got 50% more weight ( or more) into a Ju 88 sized plane it doesn't solve many of the other problems.

Contemporary Henschel pressure cabin.


In 1939/40 the Germans had NO working power turrets in service and they were planning on skipping the manned power turret and jumping to the remote control power turret. Now it is easier to make the pressure cabin that way but with little or no practical experience in turret design the turrets/barbettes are not an easy solution.

The Germans seem to have had landing gear problems, perhaps the allies had just as many and they are just not chronicled as much? But many German programs had numbers of prototypes damaged or destroyed by landing gear failures. The Ju 288 program had about 50% of the aircraft damaged/destroyed by landing gear problems, how much this was caused by compact/light landing gear trying to handle a heavy hot airplane I don't know ( and I may be reaching). The A-26 had a wing loading comparable to the Ju-288 but it took the US 3 steps to get there. The A-20/B-25 followed (Not by much) by the B-26 and then the A-26 which used about the same engines as the B-26 but in a smaller airplane. The A-26 compensated to some extent by being the first plane in service with double slotted Fowler flaps ( earlier planes had used single slotted Fowler Flaps).

It may be pure speculation on my part but perhaps the RLM wanted bigger wings to help with field performance? Much like the Martin B-26 had both a larger wing and a change in wing incidence to help with field performance although the USAAC was willing to sacrifice flight performance to get it ( and not demand more powerful engines to get the flight performance back)

I have seen: Junkers Jumo 222

and I do have a copy of the magazine with the article referred to "Torque Meter Vol. 6, No. 3 (Summer, 2007)"

Which, while offering a 14 page article with lots of pictures of parts and a detailed description of several Jumo 222A/B and E/F engines torn down at Wright Field in 1946 in preparation for building a running example, is a bit skimpy on the history of the engine. Engines were obtained from the Navy engine test station in Philadelphia. Where they got them it not stated ( direct from Germany or engines in British possession handed over ?) Article claims 289 built but gives no source or reference for this. and no break down is given of how many of each type was made or when, which to my mind is one of the big unanswered questions so far.

1. how many were built, the 289/291 number ( and I don't care about 2 engines) or a significantly smaller number?
2. Which models and when?
If there were even a couple dozen A?B engines sitting around in 1942/43 and they were good for 30-50 hours why weren't they being used for prototype flying? If the large number built was E/F version in 1944 and due to bombing and production schedules being jerked around (and bombing) it is a bit more understandable why the didn't get into some sort of aircraft.

One interesting feature is that while the engine used two magnetos and had dual plugs each magneto ran 1/2 the engine. One magneto supplying ALL the plugs in 3 banks of cylinders so a magneto failure takes out 1/2 the engine.
 
AFAIK the landing gear issues were worked out by the end of 1942 along with the structural issues; the last remaining problem by 1943 was engines. Part of the landing gear problems had to do with the increase in weight between the A-series and B-series, which required time to work out (there were also some issues during the up-weighting between A-series prototypes too). Had they stuck with the A-series, then the landing gear issues would have been worked out sooner.

Beyond that you make a solid point about the remote controlled defensive armament; it probably would have been easier to simply remove them once they proved too advanced for the time to simplify construction until they could be further developed and rely on speed and escorts to defend them.

BTW does anyone know what that fourth crew member that was added to the B-series actually did? I can't find information about it.
 
Regarding the remote controlled turret.

The FDL-B 131/2 with FA-15 appearently never made big issues. The same system for waist guns drove the Me-210 and Me-410 and a derived system similar to the B stand was employed in the Ar-240 and He-177 B-1 stand and other Bomber-B projects had them too. Post war analysis showed the system to be mechanically reliable and offering better arcs of fire. It allowed for concentration of the crew in a better supportable compact space.

In context to the Ju-288 it´s not known to me that problems with the RPC turrets in any form delayed the project.
Am not sure how this can be considered too advanced for the time. Heinkel, Junkers, Messerschmidt and Focke Wulf went down this path starting in the late 30´s and by the late 40´s, everybody regularely employed them.


It´s obvious that they had a number of landing gear failures. This was aggreviated by the fact that the relatively high stall speed required harder touch down conditions. It was further aggreviated by the repeated weight increases which required fixes to the mechanical design. However, in the course of the testing phase these issues have been largely resolved and wouldn´t have been that bad had they agreed upon the original design.


There were a couple of reasons:

The original small wing of the EF-78 proposal was found to be deficient in the V-1 by fligth testing due to comparably poor transverse and directional stability. In response, the fuselage was lengthened and the tailsurfaces reshaped. This adressed directional stability and improved transverse stability. in July 1940, the RLM following experiences at the BoB made substantial changes to the performance requirements:
Much increased defensive armement (requiring a fourth crew member) and upped max bombload by 1t. Thus, it became necessary to add two wingstations for bombs, since the internal bombbay couldn´t have carried 6t. It also required an entirely new cockpit design.
This in turn forced to insect a small new fuselage section for the lower RPC turret and an entirely new wing with 5m^2 bigger area to keep wingload down despite the massive increase in MTOW. It meant that the JUMO-222 with 2000Hp rated military power was now found a bit on the low side of performance. The take off power of 2,500Hp was required, resulting in a shift in development from JUMO-222A-1 to JUMO-222A-2 with enlarged bore. It also meant that the JU-288V05 couldn´t be used as a template for the proposed A-1 series anymore, resulting in the all-new B series (which only kept the center fuselage sections of the former A subtype).
The B-series may also have been intended for high level bombing principally (no dive bombing requirement anymore) and higher ceiling requires bigger wings, too.
The shift from -A1 to A-2 subvariant of the JUMO-222 was unfortunate as it entailed harmonic issues which soon delayed the program and required constant care. While the performance was successfully increased (in mid 1941 the engine first run at 2,400hp and in oct that year made short time power of 3,000HP on C3-fuel), several mechanical issues appeared which prooved difficult to overcome. Spacing of the enlarged cylinders was problematic, too, requiring substantially more heat transfer to be done than in the A1 subtype. In the end, the A-3/B-3 was the fix for the harmonic issues encountered in the A2/B2. The A1/B1 on the other hand convinced by it´s smootheness of run and these problems weren´t alltogether anticipated to be caused by a slight change in bore.


If there were even a couple dozen A?B engines sitting around in 1942/43 and they were good for 30-50 hours why weren't they being used for prototype flying?
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I think they were used. The V05 alone supposedly made 1200 hours to nov 1943. This requires 2400 engine hours or about 12 engines with appox. 100 hours each between MOT and supplied back a second time. It actually was more because a couple of them were wrecked in the testing program.
I also dont know how many JUMO-222 were produced by Germany. At least the A-1 subtype was completely developed. max. sustainable power was 1850hp with 2,000HP military power and 2,500Hp take off/emergancy power (the latter never cleared on B4 fuel). A complete preseries of 50 engines were manufactured in addition to an unspecified number of prototypes. Preparation of mass production at MMF and Ostmark proceeded until late 1940 alongside preperation of mass produtction of the JU-288A1. Junkers already made over 6000 specialised tooling machines for it and all involved suppliers. From the A2/B2 and A3/B3 engines I have seen no data but according to primary sources, Milch decided against mass production of these engines in late 1942 because he didn´t wanted the Junkers company to be in charge of so many important dev engine projects (JUMO-213, JUMO-222, JUMO-004) he somehow feared overconcentration of important projects in just one company. The C3 fuel optimised C/D subtypes were just drawing board projects with one or two prototypes buildt (but not tested) and the E/F proceeded at low pace from prototype to pre-series production.
In Oberursel, a licensee, test stands were erected for JUMO-222E/F in early 1944 but only five JUMO-222E were produced here before the Jägernotprogramm killed all developmental non-jet engines for non-fighters from production lists in 1944. The -222E/F saw a rather widespread preparation for mass production in 1944 and a number of engines have to be reckoned to be from this subtype.
According to Reinhard Müller and Holgar Lorenz´s accounts, the A1/B1, A2/B2 and A3/B3 were produced in V-numbers and preseries production as was the E/F high altitude variant (if You take the A-1/B-1 number of 50 engines this makes for 200 engines not accounting for prototypes). The C/D was only in prototype stage and the -T subvariant a prototype benchtested. Thus, a total production number of in between 250 and 300 JUMO-222 appears to me as entirely believable.
Wörth mentioning, there were a couple of JUMO-222 produced after the end of war for the soviets in Köthen, too.
 
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