Two engine R-3350 powered bomber

[FLYBOYJ]

An ASW plane, that flies most of the time practically on the deck, needs different engines and/or engine settings than a bomber or an airliner flying mostly above 20000 feet......

Actually yes and no...

The P-2 and later P-3 as we know spend a lot of time on the deck, but to get "on station" they flew at higher altitudes, it was only during patrols that you saw them down low, and this was probably emphasized in post war ASW doctrine. I do know the P-2 didn't have a great service ceiling, about 20K IIRC.

 

[gjs238]

Actually yes and no...

The P-2 and later P-3 as we know spend a lot of time on the deck, but to get "on station" they flew at higher altitudes, it was only during patrols that you saw them down low, and this was probably emphasized in post war ASW doctrine. I do know the P-2 didn't have a great service ceiling, about 20K IIRC.

The PB4Y-2 Privateer's were made sans turbochargers.
(Not arguing, just sayin')

 

[Shortround6]

Everything is a tradeoff. Was the faster transit time the turbo offered worth the weight of the turbo installation in fuel for a given gross weight
A plane with 2 speed supercharger could cruise at decent heights to begin with. Lockheed Constellations didn't have turbo. Of course they weren't loaded down like some bombers were either.

 

[gjs238]

Everything is a tradeoff. Was the faster transit time the turbo offered worth the weight of the turbo installation in fuel for a given gross weight
A plane with 2 speed supercharger could cruise at decent heights to begin with. Lockheed Constellations didn't have turbo. Of course they weren't loaded down like some bombers were either.

oh...was thinking of the turbos more for altitude performance than speed/faster transit time.

 

[Koopernic]

Actually yes and no...

The P-2 and later P-3 as we know spend a lot of time on the deck, but to get "on station" they flew at higher altitudes, it was only during patrols that you saw them down low, and this was probably emphasized in post war ASW doctrine. I do know the P-2 didn't have a great service ceiling, about 20K IIRC.

During the (actually still on going) search for Malaysia Airlines MH370 which was thought to be lost over 2500km/1550 miles from Australia's West Coast the problem of transit times to and from the search area became significant for RAAF P3C. The Boeing 737 based P8 (USN or USAF?) that became involved acquitted itself quite well because of its greater transit speed.

 

[FLYBOYJ]

During the (actually still on going) search for Malaysia Airlines MH370 which was thought to be lost over 2500km/1550 miles from Australia's West Coast the problem of transit times to and from the search area became significant for RAAF P3C. The Boeing 737 based P8 (USN or USAF?) that became involved acquitted itself quite well because of its greater transit speed.

The P-8 is operated by the USN.

It's obvious a patrol plane based on the B737 is going to have a quicker "on station" capability than a turbo prob. The P-3 will excel in loiter time. Although the P-8 is based on a modern airframe, I believe there are liabilities in choosing a turbofan powered aircraft for an ASW mission, but then again being a former P-3 maintainer, I will admit my bias.

 

[kool kitty89]

Good summary (of the heavy bomber part of the query), thanks!

The second part of the question concerned a higher performance medium bomber.
There has been much discussion (on this board) of the concept of allied "schnell bombers" in the ETO and elsewhere.
For example, we frequently discuss the use of swarms of Mosquitoes in lieu of armed heavy bombers.
Perhaps a 2-engined R-3350 craft could fit into that discussion.

If speed is the issue (maneuverability not a primary factor). Fast heavy bombers might be feasible too. Designs similar to the B-17 and B-24 but designed with no provision for little/no defensive armament and more emphasis on drag reduction. (the experimental V-1710 powered B-17 derivative proved substantially faster than the R-1820 powered one -fan cooled radial configurations might have been possible too or even 4 engined R-2800 powered bombers)

Turbocharged R-2800 powered medium bombers with an emphasis on speed should have been practical. And light/medium bombers with R-1820/1830 or V-1710s should have been possible too. (or a 'light' bomber with the bomb capacity of a medium bomber - a la Mosquito) Something like a turbocharged B-25, B-26, or A-26. (or maybe A-20)

 

[Shortround6]

If speed is the issue (maneuverability not a primary factor). Fast heavy bombers might be feasible too. Designs similar to the B-17 and B-24 but designed with no provision for little/no defensive armament and more emphasis on drag reduction. (the experimental V-1710 powered B-17 derivative proved substantially faster than the R-1820 powered one -fan cooled radial configurations might have been possible too or even 4 engined R-2800 powered bombers)

OK, you have three if not four rather different suggestions in one sentence.

1. The Lockheed P2V-1 carried only six .50 cal guns so it did have marked reduction on defensive armament from B-17s or B-24s. Ditching the nose mount and sticking on a more pointed streamline nose did help some.
2. There is a lot of controversy over the Allison powered XB-38. Difference in performance was not anywhere near as great as many people make it out to be (it was a converted "E" series and not the heavier/dirtier "G" series) and how much of the improvement was due to using 1425 engines vs 1200hp engines is certainly open to debate. Weight for performance figures is also open to question (are the numbers for the radial powered "E" and "F" done at a lower than normal operational weight?).
3. Fans seem to have rather overblown reputation. Used basically to help cool off radials while taking off or climbing when the plane can't get enough airflow through the cowling due to the cowling being too small or having too small openings.
This was 'solved', at least somewhat, on some aircraft by using either modified prop-blades or cuffs.

One of the few US production aircraft to use fan cooled engines in WW II (helicopters are another story) was the Martin PBM flyboat. Several hundred of them got fan cooled 1900hp R-2600 engines, Cooling problem (and low power) was solved in the next production batch by using P W R-2800s without fans.
4.A bomber with four R-2800s is more like 4/5ths B-29 than a direct replacement for the B-17/B-24.

Turbocharged R-2800 powered medium bombers with an emphasis on speed should have been practical. And light/medium bombers with R-1820/1830 or V-1710s should have been possible too. (or a 'light' bomber with the bomb capacity of a medium bomber - a la Mosquito) Something like a turbocharged B-25, B-26, or A-26. (or maybe A-20)

You need a lot more R-2800 engines.
The US wasn't interested in light/medium bombers with R-1820/1830 or V-1710s. They had seen the A-20 with R-1830s. The US wasn't interested in short range aircraft with limited bomb loads. The Continental US is too big to make such planes practical and with the US overseas possessions (like the Panama Canal) only adding to the problem of range. The US also had to ship (literally use ships) most of the smaller aircraft to combat theaters. Large numbers of low capability aircraft vs fewer but larger and more capable aircraft didn't make sense.
The Mosquito didn't display it's "capacity of a medium bomber" until the Spring of 1943. Roughly 10 months after the A-26 first flew. A little late to start programs ore even modify programs to any extent and get any numbers of aircraft (more than a squadron or two) into service before the wars end.
The XB-28 did use Turboed R-2800s. The USAAF had decided that medium bombers bombing from high altitudes wasn't going to work well. Something that also killed the Turboed A-20. Along with the fact that early R-2600s didn't seem to take to turbo charging very well. Early R-2600 powered A-20s had some cooling problems and adding the turbo didn't help.

 

[gjs238]

4.A bomber with four R-2800s is more like 4/5ths B-29 than a direct replacement for the B-17/B-24.

Another interesting concept to explore.
We often say that lots of good enough now is better than waiting for perfection to arrive after it's too late.
Or something like that.

The point is, perhaps 4/5ths B-29 could have entered service sooner and had a greater impact on the war?
Don't want to be too quick to draw conclusions, rather it would be more fun to explore the idea.

 

[Shortround6]

Part of the problem with this is figuring out what delayed the B-29 and if switching to R-2800s would change anything. You also have to decided by late 1941 which engine you are going to use and build the appropriate manufacturing plant in order to get engines by early 1944. (that or steal R-2800s form other aircraft).

"On May 17, 1941, the Army announced that an order would be placed for 250 B-29s to be built in a new government-owned facility at Wichita, Kansas that would be leased by Boeing for B-29 production. This order was confirmed in September of 1941."

"The first XB-29 (41-0002) flew on September 21, 1942 ....... By this time, there were 1664 B-29 aircraft on order."

Quotes are from Joe Baugher's website.
The XB-29 was in instant trouble with the engines but how much trouble did they have with the remote control gun systems? (first prototype had no guns.) How much trouble with the Pressurization?

" After tests with the Sperry system of retractable turrets and periscopic sights on the first three XB-29 prototypes, the Sperry contract was withdrawn and given to General Electric."
"This new remotely-controlled armament system was first installed in the third XB-29. Unfortunately, the new system required a lot more electrical power, necessitating the addition of several specially-designed generators. This delayed the onset of B-29 production still further, and brought the gross weight of the aircraft to over 105,000 pounds"
Quotes again from Joe Baugher's website.

Early B-29s suffered from being under powered so you just can't swap engines. You need a different/smaller airframe. The question is do you want a "safe" low tech airframe (unpressurized and using existing defensive armament turrets) or do you want the 'advantages' promised by the high tech B-29?

You will get different speed and range numbers depending on which option you pick.

 

[Macchi]

A little off topic, and I know the engine wasn't ready in WWII, but one of my warplane fantasies would be a Beaufighter with two Pratt Whitney R-4360 engines.

 

[kool kitty89]

3. Fans seem to have rather overblown reputation. Used basically to help cool off radials while taking off or climbing when the plane can't get enough airflow through the cowling due to the cowling being too small or having too small openings.

Small/tight cowlings better suited to high-speed aircraft, but less useful for low/moderate speeds. (particularly in the >350 MPH or >.5 mach range)

For lower speeds, the added weight, complexity, and power loss at low speeds makes it unattractive compared to similarly well designed cowlings with larger area. (and cuffed propellers)

Most installations of the BMW 801 likely benefited from its fan and cowling configuration, granted some may not have. (the advantage there was standardized production and modular installation -plus the fan was geared independently of the propeller itself)

The US wasn't interested in light/medium bombers with R-1820/1830 or V-1710s.

They also weren't generally interested in unarmmed fast bombers of any size ... regardless of range and bombload capabilities.

A mosquito-like bomber using turbocharged V-1710s would have been pretty interesting. (or even Ju-88 like ... without the defensive armament and dive bombing being mandated)
Then again something slightly larger using R-2800s might have been more useful on the whole. (perhaps even something similar tot he B-17 or B-24 mission profile in terms of range and bombload, but smaller, faster, and lighter, and with less crew)

The XB-28 did use Turboed R-2800s. The USAAF had decided that medium bombers bombing from high altitudes wasn't going to work well. Something that also killed the Turboed A-20. Along with the fact that early R-2600s didn't seem to take to turbo charging very well. Early R-2600 powered A-20s had some cooling problems and adding the turbo didn't help.

It's not just a matter of high altitudes at the levels B-17s and B-24s usually cruised at, but anything above medium altitudes, at least if you wanted economical cruise power and fuel consumption. (and the American heavies may have been too vulnerable when dropping their altitude a few thousand feet to improve accuracy, but this would have been different if they were flying close to 100 MPH faster)

As it was, the Halifax and especially Lancaster were significantly faster at low/medium altitudes than American heavies ... something closer to that with turbocharged -or possibly efficient high altitude supercharged- engines could have been significantly faster. (let alone purpose designed to rely on high speed and no defensive armament)

Early B-29s suffered from being under powered so you just can't swap engines. You need a different/smaller airframe. The question is do you want a "safe" low tech airframe (unpressurized and using existing defensive armament turrets) or do you want the 'advantages' promised by the high tech B-29?

You will get different speed and range numbers depending on which option you pick.

And they'd already swapped the engines with the R-4360s not maturing in time and the R-3350s used in the interim. Part of the reason it was initially underpowered.

Another interesting concept to explore.
We often say that lots of good enough now is better than waiting for perfection to arrive after it's too late.
Or something like that.

The point is, perhaps 4/5ths B-29 could have entered service sooner and had a greater impact on the war?
Don't want to be too quick to draw conclusions, rather it would be more fun to explore the idea.

I think the point here would be less a literal 4/5ths of a B-29 and more a totally different purpose-built aircraft intending to use 4x R-2800s either a similar, but somewhat smaller alternative to the B-29 (or B-32) or a more radical design focusing on the unarmmed (or nearly unarrmed) concept. The only thing remotely close to that in development during WWII in the US was Northrop's Flying Wings. But ... something consdierably more conservative in as far as conventional wing/body configuration but still aiming at high speed and light or no armament (and targeting lower power engines) would have been significant.

 

[Koopernic]

As far as using the R-2800 in lieu of the R-3350 for the B-29, I think it would be possible. PW R-2800 with water injection were producing 2200hh-2300hp in the P-47 by 1943, 2600hp by 1944 and 2800hp before 1945 on the P-47M. The question would arise as to whether the R-2800 overboosted with ADI (Anti Detonation Injection) of Water and Alcohol would work for a bomber in slowly accelerating take-off roll when there was less airflow versus a P-47 using it for an extra burst of speed.

My suspicion is that it would work so long as the same techniques as were eventually used with the B-29: a rolling start during which spark plug test were performed, speed and therefore airflow was built up prior to full power being applied. From the point of view of take of run the acceleration in the second half the takeoff run is by far the most important.

The R-3350 would only offer higher sustained power but in terms of maximum power offered no advantage untill the post war period.

It's worth looking at the saliant parameters of these radial engines in terms of diameter.

CW R-2600 cubic inches, 55 inches, 1700hp, 2045lbs
PW R-2800 cubic inches, 52.8 inches, 2000hp, 2360lbs
CW R-3350 cubic inches, 55 inches, 2200hp, 2670lbs
Bristol Hercules IV:
R-2360 cubic inches, 55 incches, 1650hp, 1929lbs
BMW801
R-2560 cubic inches, 51 inches, 1700hp, 2226lbs (low grade C3 fuel produced 1700, high grade equal to 100/130 1900hp, weight includes fan)
As can be seen the R-3350 is the same diameter as the R-2600 (55 inches) while the R-2800 is actually smaller at 53 inches. (The BMW801 really stands out at only 51 inches.)
This is why the R-2800 so quickly 'stole' the market the R-2600 created.

Hence any aircraft built for the R-2600/R2800 should accomodate the R-3350 so long as increased weights and center of gravity considerations are made. We would need to account for 15% more weight for the bigger engine and another 15% more for the turbo installation.

My argument would be that a direct replacement of the R-2800 in the B-26G marauder with the turbo charged R-3350 as used in the B-29 would produce a potent bomber within the limits of usuabillity so long as a good runway was provided.

I recall reading in "Wings Magazine" that a turbo charged Curtiss Wright R-2600 was at one time considered for the B-26 Marauder and that with this engine top speeds of 400mph were estimated. For some reason work on this engine was abandoned.

Fitting the more powerful R-3350, even in consideration of the increased weights and enlarged wings of the B-26G, should give a speed in excess of 400mph at high altitude (say 25000ft). I imagine one would drop the 4 fixed forward firing guns to compensate centre of gravity changes and reduced drag. If more tail weight is needed the rear gunner can be given 5 inches of armour glass and a 12.7mm steal plate. The engine might need a 5 blade propeller for the sake of ground clearance. The turbo charged R-2800 could do the job as well but the R-3350 would provide about 15% perhaps 25% more power unless the water injected WEP ratings of the R-2800 are considered, in which case the R-2800 is more powerful.

A Mediium bomber able to excede 400mph at 20,000-25,000ft and perhaps cruise at 370mph would be a tough machine to intercept and shoot down. Higher ceilings possible though the lack of pressurisation might be an issue. By restricting modifications to the minium production should be affordable and feasible. This aircradt would not perform the B-26 role of bombing accuratly from 8000-12000ft to take out a bridge etc but be an impossible to intercept medium bomber to frustrate the enemy.

Historically the USAAF did ask for advanced pressurised improvements to the B-26 and B25. These were the Martin XB-27 and the North American XB-28. Both looked like a Marauder.

 

[gjs238]

I think the point here would be less a literal 4/5ths of a B-29 and more a totally different purpose-built aircraft intending to use 4x R-2800s either a similar, but somewhat smaller alternative to the B-29 (or B-32) or a more radical design focusing on the unarmmed (or nearly unarrmed) concept. The only thing remotely close to that in development during WWII in the US was Northrop's Flying Wings. But ... something consdierably more conservative in as far as conventional wing/body configuration but still aiming at high speed and light or no armament (and targeting lower power engines) would have been significant.

Bingo!

 

[Shortround6]

Small/tight cowlings better suited to high-speed aircraft, but less useful for low/moderate speeds. (particularly in the >350 MPH or >.5 mach range)

For lower speeds, the added weight, complexity, and power loss at low speeds makes it unattractive compared to similarly well designed cowlings with larger area. (and cuffed propellers)

Sometimes it was just extreme service.

View attachment 286828

It was easier to add a fan (which might have turned at propeller speed) than redesign cowl when they went to the 1900hp engine.

They also weren't generally interested in unarmmed fast bombers of any size ... regardless of range and bombload capabilities.

True but then the Mosquito had not proved itself BEFORE ANY US Bomber that saw service in WW II started design work.

The Army had sponsored a Design competition in 1938 for a light bomber/attack aircraft that saw 4 different aircraft built. The Douglas DB-7B, North American NA-40, the Stearman X-100 and the Martin 167F. 3 used P&W R-1830s and the Stearman used P&W R-2180s. ALL flew in 1939. The Army didn't like the results and started a new "competition" which was won by a modified DB-7, North American modified the NA-40 into the B-25 for a different requirement. Martin sold the 167 to the French and British as the Maryland. The Army had a pretty good idea of what kind of performance it could get from an R-1830 powered twin engine bomber and they weren't interested. A twin Allison powered bomber (aside from facing an intimidate engine shortage) offers less drag but not much else. Please remember that the Mosquito was planned as a light bomber with a payload of four 250lbs.
A-20s and B-25s were going into service with US squadrons 9 months to year before a Mosquito dropped a bomb in anger. The summer of 1942 is not the time to try switching production lines.

A mosquito-like bomber using turbocharged V-1710s would have been pretty interesting. (or even Ju-88 like ... without the defensive armament and dive bombing being mandated)
Then again something slightly larger using R-2800s might have been more useful on the whole. (perhaps even something similar tot he B-17 or B-24 mission profile in terms of range and bombload, but smaller, faster, and lighter, and with less crew)

Interesting yes, but interesting isn't the same thing as useful. And I am still having trouble seeing the JU-88 as some kind of wonder plane. 25mph slower than an A-20 (the JU-88A-4), Range isn't much better, if any better, both had a variety of fuel tank set ups so make sure you are comparing the right things. The Ju-88S-1, so beloved by advocates of fast bombers, shows up in the Autumn of 1943, only a few months ahead of the A-26. Not much of a contest as to which was the better bomber.
Switching from turbocharged V-1710s to R-2800s doesn't result in a "slightly larger" airplane. You have added well over a ton of engine weight and more like 1.5-2 tons of powerplant weight. Powerplant weight (for piston engines) went about 30-40% of all up weight (clean) for fighter type aircraft. For "light " bombers it is a bit less. Throw in the fuel you need to feed the R-2800s and AUW goes up in hurry. Again lets compare apples to apples. A-26 used an air foil and flap system that didn't exist in 1939-40. The A-26 could have been speeded up a bit but you weren't going to get them in service in 1942 without a time machine.

More later.

 

[Shortround6]

As far as using the R-2800 in lieu of the R-3350 for the B-29, I think it would be possible. PW R-2800 with water injection were producing 2200hh-2300hp in the P-47 by 1943, 2600hp by 1944 and 2800hp before 1945 on the P-47M. The question would arise as to whether the R-2800 overboosted with ADI (Anti Detonation Injection) of Water and Alcohol would work for a bomber in slowly accelerating take-off roll when there was less airflow versus a P-47 using it for an extra burst of speed.

It is not just take-off but climb out and cruise. The max continuous power ratings (normal?)(climb?) of the R-2800 engines didn't change as the power rating went up until the "C" series engines showed up. The "B" series engines with turbos were good for 1625hp as long as the fuel held out and the temperature gauges stayed in the safe zone. The early "C" series engines went to 1700hp. The 2200hp take-off rated R-3350s were rated for 2000hp Normal.
Change over from rich to lean mixture settings would also see a several hundred HP advantage for the the double Cyclone. It could run lean and make more power than an R-2800 running a rich cruise setting. R-2800 with turbo gave 1200hp max running lean.

From a Manual on the B-29 it took 29 minutes, 580gallons of fuel and 120 miles to climb from 14,000ft to 25,000ft using 2000hp per engine at 120,000lbs. Using 1760hp per engine the correction factor was the same as adding 16,000lb to the airplanes weight. IF I am reading the chart right (BIG IF) that could mean using almost twice the fuel to make the same climb. 2000hp per engine used 91 pounds of fuel a minute and 1760hp used 74 pounds a minute but the climb from 14,000ft to 25,000ft (if you could even make 25,000ft using 1760hp per engine ?) is going to take over 60 minutes.
BTW it took about 21-22 minutes to go from sea level to 14,000ft and 400 gallons of fuel.

Doesn't matter if you can blast the B-29 off the runway using water injected R-2800s if you can't even get the plane to operational altitudes without totally abusing the engines.
If you lighten the plane up to about 100,000lbs you just might make it.

 

[wuzak]

You could, perhaps, run 6 R-2800s on a B-29.

 

[kool kitty89]

As far as using the R-2800 in lieu of the R-3350 for the B-29, I think it would be possible. PW R-2800 with water injection were producing 2200hh-2300hp in the P-47 by 1943, 2600hp by 1944 and 2800hp before 1945 on the P-47M. The question would arise as to whether the R-2800 overboosted with ADI (Anti Detonation Injection) of Water and Alcohol would work for a bomber in slowly accelerating take-off roll when there was less airflow versus a P-47 using it for an extra burst of speed.

Maximum power output of the R-2800 series was sufficient for take-off and emergency power, but the problem comes with cruise power. Either you'd have to run at lower power levels to reach good fuel economy (but suffer lower cruise speeds) or sacrifice range by operating beyond maximum cruise power. (and not being able to use lean mixtures)

At least that's the appraisal that came up years ago when this was posed. I think it was Delcyros who posted the comparitive fuel consumption. (there may have been other factors not addressed, like cruise power setting not typically/officially documented in the P-47 manuals being referenced -reluctance to employ high boost pressure at low RPM and lean mixture was a major issue for the P-38's performance, not sure if this was delayed/overlooked on the P-47 as well)

Shortround covered this well already above, too.

This is why the R-2800 so quickly 'stole' the market the R-2600 created.

There's other issues like power output, supercharger/turbocharger developments and compatibility, and overall development path of the R-2800 being smoother. (the R-2600 had some engineering problems, some related to the lead designer being taken off the project due to German heritage, I believe)

The R-2600 was slightly wider in diameter, but significantly lighter than the R-2800 and possibly mechanically simpler in some respects. With smoother development progress it might have been more competitive than it was. (as it was, it managed to be farily competitive -or superior to- contemporary Bristol Hercules engines)

I recall reading in "Wings Magazine" that a turbo charged Curtiss Wright R-2600 was at one time considered for the B-26 Marauder and that with this engine top speeds of 400mph were estimated. For some reason work on this engine was abandoned.

Most prototype turbo installations with R-2600s appear to have been unsatisfactor for one reason or another, I'm not sure why but it's one of the reasons the R-2800 was more successful.



The problem with the R-3350 in general is similar to that of the Bristol Centaurus. It's a late-war design that had barely matured by the end of the war. Granted, the R-3350 progressed well enough to be pressed into service in fairly large numbers on B-29s, unlike the Centaurus.

 

[GrauGeist]

The problem with the R-3350 in general is similar to that of the Bristol Centaurus. It's a late-war design that had barely matured by the end of the war. Granted, the R-3350 progressed well enough to be pressed into service in fairly large numbers on B-29s, unlike the Centaurus.

The R-3350 was a pre-war design.

It certainly had a lot of "teething" problems, (which were resolved towards the end of the war), so much so, that the XB-39 was developed as an alternative (powered by the Allison V-3420-11) to the B-29 in case the R-3350 didn't work out.

 

[kool kitty89]

A-20s and B-25s were going into service with US squadrons 9 months to year before a Mosquito dropped a bomb in anger. The summer of 1942 is not the time to try switching production lines.

Interesting yes, but interesting isn't the same thing as useful. And I am still having trouble seeing the JU-88 as some kind of wonder plane. 25mph slower than an A-20 (the JU-88A-4), Range isn't much better, if any better, both had a variety of fuel tank set ups so make sure you are comparing the right things. The Ju-88S-1, so beloved by advocates of fast bombers, shows up in the Autumn of 1943, only a few months ahead of the A-26. Not much of a contest as to which was the better bomber.
Switching from turbocharged V-1710s to R-2800s doesn't result in a "slightly larger" airplane. You have added well over a ton of engine weight and more like 1.5-2 tons of powerplant weight. Powerplant weight (for piston engines) went about 30-40% of all up weight (clean) for fighter type aircraft. For "light " bombers it is a bit less. Throw in the fuel you need to feed the R-2800s and AUW goes up in hurry. Again lets compare apples to apples. A-26 used an air foil and flap system that didn't exist in 1939-40. The A-26 could have been speeded up a bit but you weren't going to get them in service in 1942 without a time machine.

The A-20 itself seems like it might have fared well enough in a mosquito-like role, though the internal bombload was more limited. (more limited than the Ju-88's maximum internal load too, though not limited to the small, vertically oriented bombs as on the Ju-88 )

Swapping Allisons for R-2800s would indeed be a bit of a different matter entirely, a major design shift. Switching from R-2600s to R-2800s would be less dramatic, though more so if consider 2-stage or turbocharged R-2800s. The dimensions and weights of the A-20 are fairly close to the F7F ... if obviously not as clean of an aircraft. Still, it might have already been close enough to be modified (without a total redesign) into an unarmmed fast-bomber configuration with more powerful engines. (logistics of turbocharger supplies or 2-stage R-2800s would have limited matters, though, and just stripping down the existing A-20s probably wouldn't have gained enough performance to be really good 'fast' bombers)

The B-25 or B-26 might have had some potential in that role too, and certainly were more competitive with the Mosquito in terms of internal load, but higher powered 2-stage or turbocharged R-2800s might not have been enough there. (but that's back to the R-3350 hypothetical)

The R-3350 was a pre-war design.

It certainly had a lot of "teething" problems, (which were resolved towards the end of the war), so much so, that the XB-39 was developed as an alternative (powered by the Allison V-3420-11) to the B-29 in case the R-3350 didn't work out.

I was referring to maturity as a reliable production design. I'm not really sure the V-3420 would have been a better alternative there either given its own maturation cycle.