Vulnerability of liquid cooled engines

[Greg Boeser]

Except, of course, The defenseless bombers would suffer higher casualties as the defending fighters would have no return fire to contend with, which means that they can press their attacks closer and longer. Plenty of reports of bomber formations being stalked by fighters that remained just outside effective gun range, that might make one or two half-hearted passes without risking return fire. Can this be said for unarmed bombers? I think not. The only thing to end a fighter attack on an unarmed bomber is the limit of the fighter's range/endurance and ammo.

 

[Milosh]

or escort fighters.

How many Mosquito bombers were shot down by fighters?

 

[Shortround6]

I wonder if a bomber designed from the start with no defensive armament would have suffered overall less losses, a cruising speed near 300MPH would have given night fighters and flak a bigger problem.

It might have but trying to cruise at 300mph is not easy for a large plane, especially one designed in the late 30s. Granted a 4 engine "fast" bomber would not have to keep the legacy fuselage of the Lancaster and Halifax if reduced to a 3-5 man crew.

However whatever plane is being designed in the late 30s has to operate out of the existing airfields or the size of the airfields in the foreseeable future,
NOT the airfields that would exist in 1944-45. This means the thick, high drag wings (Fowler flaps and other aerodynamic trickery helps a lot more on landing than taking off) cannot be totally abandoned.

For Example a B-24 ( Liberator VI) needed 2100 yds to take-off and clear 50ft at 62,000lbs. A Fortress III needed 1560yds at 64,000lbs
A Halifax I (with Merlin X engines) need 1400yds at 59,000lbs, A Halifax II (with Merlin XX) needed 1200yds at 60,000lbs and a Lancaster I needed 1500 yds at 68,000lbs and 1200yds at 45,000lbs.
Everybody agrees the B-24 had the lowest drag wing, Now can you actually fly the B-24 from British 1938-39-40 airfields? even if you chop 10,000lbs out of it?

A Mosquito IV needed 840 yds at 21,462lbs using 12lbs of boost. but was only carrying four 500lb bombs.

Hooker only gets to RR and modifies the supercharger in late 1939, so any planning done in 1938-39 has to be done with the Merlin X in mind.

 

[swampyankee]

Except, of course, The defenseless bombers would suffer higher casualties as the defending fighters would have no return fire to contend with, which means that they can press their attacks closer and longer. Plenty of reports of bomber formations being stalked by fighters that remained just outside effective gun range, that might make one or two half-hearted passes without risking return fire. Can this be said for unarmed bombers? I think not. The only thing to end a fighter attack on an unarmed bomber is the limit of the fighter's range/endurance and ammo.

And the bombers' escorting fighters. I think the USAAF found, much to the dismay and surprise of its planners, that unescorted bombers, regardless of how heavily armed, were highly vulnerable to fighter attack.

 

[swampyankee]

It might have but trying to cruise at 300mph is not easy for a large plane, especially one designed in the late 30s. Granted a 4 engine "fast" bomber would not have to keep the legacy fuselage of the Lancaster and Halifax if reduced to a 3-5 man crew.

The cruise speed of the first model of the Lockheed Constellation cruised at 340 mph (295 knots) and entered service in mid-1943. It could have been done, but USAAF planners had decided that bombers needed to have heavy defensive armament.

Advocates of strategic bombing, in general, both radically overestimated the effectiveness of bombing and the effectiveness of bombers' defensive armament.

 

[tomo pauk]

...
However whatever plane is being designed in the late 30s has to operate out of the existing airfields or the size of the airfields in the foreseeable future,
NOT the airfields that would exist in 1944-45. This means the thick, high drag wings (Fowler flaps and other aerodynamic trickery helps a lot more on landing than taking off) cannot be totally abandoned.
...

(my bold)
Fine post, however I disagree with bolded part. The Fowler flaps not just increased curvature (camber?) of the wing section the spanning, they also increased wing chord and thus wing area. Fowler flaps (and derivatives) directly contibute to improvement of lift coeficient and wing loading - meaning that either an aircraft can take off from same strip with bigger payload, or that it will use shorter strip for same payload, or a combination.

 

[Shortround6]

The cruise speed of the first model of the Lockheed Constellation cruised at 340 mph (295 knots) and entered service in mid-1943. It could have been done, but USAAF planners had decided that bombers needed to have heavy defensive armament.

Do you have a source for that cruise speed?
Most of what I can find is cruise speeds somewhat lower, like 275mph on 52% power at 10,000ft or 309MPH at 19,000ft on 60% power (with a post war engine) or 327 mph at 23,000ft at 65% power (also post war engine).
Please note the Constellation first grossed 86,500lbs but post war versions soon went to 102,000lbs which makes a tremendous difference to payload and range. For example the early ones did have fuel tanks holding 4690 gallons but if full that left only 3015lbs of useful load. after filling the oil tanks you barely have enough weight for crew. Payload with 2350 gals of fuel, 200 gals of oil and and crew of 4 was 15,245lbs. But at 368 gallons an hour at 275mph (52%power) that gave a range (not radius) of 1756 miles.
I believe early Constellations had a bit of trouble with their R-3350 engines overheating?

A Constellation was not going to be able to substitute for the B-29.

 

[pbehn]

I suspect -- I've not read the actual research -- that two major factors in the increase in casualties for bombers with heavy defensive armament was the increased crew size, which resulted in more casualties per aircraft shot down and in lower bombloads. Each one of those added weight -- I'd estimate at least 1,000 lb per turret and 250 lb per non-turreted flexible gun without gunners, plus 250 lb per position for gunners. Some turrets added a great deal of drag, but leaving that aside, an aircraft with the MTOW of a B-17 without the heavy and draggy turrets could carry 7,000 lb bombs to the same distance one with all the defensive weapons could carry 4,000, which would drop the aircraft required for a given mission by 75%, that is if one needed 1000 B-17s, each with ten crew members, to fulfill a mission, you'd need 250 defenseless bombers, each with no more than four. Even if both groups lost 20 bombers, the defenseless bombers would result in 80 casualties, while the armed bombers would lose 200. I don't think it would be that bad for the defenseless bombers.

It might have but trying to cruise at 300mph is not easy for a large plane, especially one designed in the late 30s. Granted a 4 engine "fast" bomber would not have to keep the legacy fuselage of the Lancaster and Halifax if reduced to a 3-5 man crew.

However whatever plane is being designed in the late 30s has to operate out of the existing airfields or the size of the airfields in the foreseeable future,
NOT the airfields that would exist in 1944-45. This means the thick, high drag wings (Fowler flaps and other aerodynamic trickery helps a lot more on landing than taking off) cannot be totally abandoned.

For Example a B-24 ( Liberator VI) needed 2100 yds to take-off and clear 50ft at 62,000lbs. A Fortress III needed 1560yds at 64,000lbs
A Halifax I (with Merlin X engines) need 1400yds at 59,000lbs, A Halifax II (with Merlin XX) needed 1200yds at 60,000lbs and a Lancaster I needed 1500 yds at 68,000lbs and 1200yds at 45,000lbs.
Everybody agrees the B-24 had the lowest drag wing, Now can you actually fly the B-24 from British 1938-39-40 airfields? even if you chop 10,000lbs out of it?

A Mosquito IV needed 840 yds at 21,462lbs using 12lbs of boost. but was only carrying four 500lb bombs.

Hooker only gets to RR and modifies the supercharger in late 1939, so any planning done in 1938-39 has to be done with the Merlin X in mind.

It was a purely theoretical idea which would need such a change in everyones strategy philosophy and experience that it is akin to science fiction. I was thinking mainly in terms of Bomber Command. In fact the front turret was dropped on the Halifax, and the mid upper was dropped on some Lancasters. Not only did the turrets, guns, ammunition with boxes and channels, and crew weigh a lot, the structure of the airplane needed to be strong enough to bear the weight throughout the mission. The three Turrets and crew added about 1.5 tons to the weight as far as I can read so a turret less aircraft could be 2 tons lighter. The inclusion of turrets must have been a major task in design calculations, they could have used that design time to reduce drag as per a fighters design, better surface finish, retractable rear wheel etc.

The use of turrets must have been an industry in itself, not only to make the tens of thousands that were made but to service, repair and crew them. Use that investment cost and manpower to produce bombers to fighter quality finish.

On night time operations the speed and concentration of the bomber stream passing over the flak defences reduced losses, while with LW night fighters higher speed was a problem to cope with.



For daylight operations it is true that the defensive fire was significant, the raids that were considered a disaster like Schweinfurt also imposed heavy losses on the LW. However, on the deepest raids many waves of fighters were used. If the Bombers had a cruising speed similar to the escorts then the USA forces could use many more fighters on each mission. Also, on a USA bomber the total weight of all that was required for defensive fire, in weight of guns turrets armour oxygen crew and strength in the airframe amounted to the bomb load and more in many cases so fewer bombers would be needed for the same tonnage of bombs.

All theory and conjecture and pie in the sky of course but things could have been done differently.

 

[Shortround6]

Perhaps but then, due to the size/complexity of the big bombers they were always going to be a bit behind single engine fighters in aerodynamics/structure.
Trying to design a 50,000lb airplane (and the tooling to build it) is always going to take longer than a 10,000lb airplane.

The Connie was an Amazing airplane but flying the Prototype in Jan 1943 (or even several months earlier) means it is two-three years after the decision to build three factories for the B-17 and use five factories to build B-24s. Brand new factories were built to build the engines for both programs. Trying to shift such programs to new types was going to be extremely difficult.

This brings us back to what was possible when. Could you build an unarmed heavy bomber, using the knowledge of the time, that had high enough performance to keep losses to a minimum in the late 30s or 1940-42? And using airfields of the time?
Progress was moving quick. The US did have the A-26 flying in mid 1942 with one of the most advanced wings used in WW II. However even with a pair of R-2800 engines and top speed of 355mph or over it still cruised at around 280-290mph.
The NA XB-28 is also instructive.

granted it has three turrets but it used the same engines as an early P-47 and the same turbos. It was roughly 50mph slower than P-47. Top speed was 370mph and the cruise was under 300mph. Do we think that deleting the three remote gun mounts and two gunners is going to gain 30-40mph?

And that is a major problem for the "high speed" bomber. It has to cruise fast. The B-28 or A-26 went over 22,000lbs empty and well over 30,000lbs loaded (over 35,000lbs full load) so trying to accelerate from "lean" cruise to top speed (or even a high cruise) was going to take a while, time the plane is not going to have if bounced. These radial engine bombers could use 300-400 gallons an hour if cruised at rich settings and well over 500 gallons an hour(8 gallons a minute) at full power.

Liquid cooled engines may do better.


The B-29 Program had full scale mock ups being inspected in Nov of 1940, just a few months after the BoB. First prototype flew in Sept 1942, just 35 days after the B-17E makes it's first combat mission in Europe.

At what point do you make the gamble for an unarmed (or lightly armed) high speed bomber?

Depending on your enemy to make mistakes or fumble his engine programs/aircraft development is also not very good planning. Depending in 1941-42 that the Germans would NOT develop (and have in mass production) a 425-440mph single engine day fighter with four 20mm cannon by 1944 is not good planning. Depending on the Germans to be using converted bombers as night fighters or still be using Bf 110s in late 1943 and 1944 is also not good planning.
What would have happened if the Germans had lowered the compression in their engines and used higher boost?
What would have happened if the Germans had gotten two stage superchargers into use sooner?

How well would an unarmed, high speed bomber have fared against allied planes?

What kind of bomber do you need to penetrate airspace defended by P-47s or Spitfire MK XIVs?

 

[swampyankee]

Perhaps but then, due to the size/complexity of the big bombers they were always going to be a bit behind single engine fighters in aerodynamics/structure.
Trying to design a 50,000lb airplane (and the tooling to build it) is always going to take longer than a 10,000lb airplane.

The Connie was an Amazing airplane but flying the Prototype in Jan 1943 (or even several months earlier) means it is two-three years after the decision to build three factories for the B-17 and use five factories to build B-24s. Brand new factories were built to build the engines for both programs. Trying to shift such programs to new types was going to be extremely difficult.

This brings us back to what was possible when. Could you build an unarmed heavy bomber, using the knowledge of the time, that had high enough performance to keep losses to a minimum in the late 30s or 1940-42? And using airfields of the time?

Yes. See, for example, the Mosquito. Before that, the US B-9 was faster than its contemporary fighter aircraft.

Progress was moving quick. The US did have the A-26 flying in mid 1942 with one of the most advanced wings used in WW II. However even with a pair of R-2800 engines and top speed of 355mph or over it still cruised at around 280-290mph.
The NA XB-28 is also instructive.
View attachment 476194
granted it has three turrets but it used the same engines as an early P-47 and the same turbos. It was roughly 50mph slower than P-47. Top speed was 370mph and the cruise was under 300mph. Do we think that deleting the three remote gun mounts and two gunners is going to gain 30-40mph?

And that is a major problem for the "high speed" bomber. It has to cruise fast. The B-28 or A-26 went over 22,000lbs empty and well over 30,000lbs loaded (over 35,000lbs full load) so trying to accelerate from "lean" cruise to top speed (or even a high cruise) was going to take a while, time the plane is not going to have if bounced. These radial engine bombers could use 300-400 gallons an hour if cruised at rich settings and well over 500 gallons an hour(8 gallons a minute) at full power.

Liquid cooled engines may do better.

The B-29 Program had full scale mock ups being inspected in Nov of 1940, just a few months after the BoB. First prototype flew in Sept 1942, just 35 days after the B-17E makes it's first combat mission in Europe.

At what point do you make the gamble for an unarmed (or lightly armed) high speed bomber?

Depending on your enemy to make mistakes or fumble his engine programs/aircraft development is also not very good planning. Depending in 1941-42 that the Germans would NOT develop (and have in mass production) a 425-440mph single engine day fighter with four 20mm cannon by 1944 is not good planning. Depending on the Germans to be using converted bombers as night fighters or still be using Bf 110s in late 1943 and 1944 is also not good planning.
What would have happened if the Germans had lowered the compression in their engines and used higher boost?
What would have happened if the Germans had gotten two stage superchargers into use sooner?

How well would an unarmed, high speed bomber have fared against allied planes?

What kind of bomber do you need to penetrate airspace defended by P-47s or Spitfire MK XIVs?

You're correct that progress was moving quickly in the mid to late 1930s, but that progress was equally beneficial for large aircraft as for fighters. In any case, I think the main barrier to the pre-war development of an unarmed or minimally armed, fast heavy bomber was not technological but internal air force ideology, that being that the bomber will always get through without fighter escort; this assumption drove the design of US bombers. When the USAAF actually had to deal with an enemy with a powerful air force defending its cities, it found its bombers wouldn't get through without unsustainable losses (I think they also found that one or two raids wouldn't cause an enemy to sue for terms).

Designing a lightly armed or unarmed strategic bomber would require the concomitant design of a high-performance, long ranged escort fighter. (No doubt somebody will blame Congress, but before doing, they should remember that the USAAF leadership was politically well-connected enough so that Congress would generally follow the USAAF's priorities; the pot of money may not have been infinite, but the USAAF had a great deal of influence over how their share of that pot was distributed, and they didn't distribute it to long-ranged escort fighters) Here, I think that the Luftwaffe, with its flawed zerstorer concept was actually considering enemy fighters as a serious threat to its bombers, showed somewhat better planning than did the USAAF. The USN, which included fighters in its carrier aviation planning, was also considering enemy fighters were a problem -- those fighters were there, at least partly, to prevent enemy fighters from decimating attack aircraft -- also showed a greater level of foresight.

 

[BiffF15]

Swampyankee,

I think it could be said they (Germany, USN, and USAAC) all had foresight. Hindsight has shown some was more accurate than others.

Cheers,
Biff

 

[Koopernic]

The early B17s seemed capable of up to 318mph. Even as engine performance increased the aircraft slowed down. The engine performance increase was mainly due to a higher critical altitude from turbo charger limits but Short term power also increased from 1200hp to about 1380ph.

So I thought strip of the dorsal turret and ventral ball turret. Tidy up the nose to B-26 standards and I think you would have quite a fast aircraft at 25,000ft to 30,000ft. We've also removed thousands of pounds of weight. Not only the turrets but it's supporting structure and also armour.

We can still retain the tail gunner, a single nose gun, maybe the radio room gun.

It's probably possible to do a speed calculation from the drag of the two deleted turrets since the can be treated as a sphere.

So I did. I modeled the ball and dorsal turret as a 1 square meter area sphere with A Cd of 0.5. At 312mp at about 28000ft the turrets absorb 343kW/450hp. Assuming 70% prop efficiency that's about 480kw/630hp at the shaft.
So getting rid of the turrets is equal to 20% increase in power for a B17E/F which 4800hp total. The cube root of 1.2 is 1.06 so we might be able to get our B17 up by 6% or 330 mph. Perhaps more with a tidy up and maybe Allison's another 10mph. So 340mph perhaps 350mph. I'd imagine maximum cruise would be about 310mph.

An Me 109G6 could manage 380mph at that altitude, 28000ft.

To me it sounds plausible and a worthwhile trade off especially if escorts can be provided.

 

[Shortround6]

Yes. See, for example, the Mosquito. Before that, the US B-9 was faster than its contemporary fighter aircraft.

A nice picture which shows the problem rather nicely.
IF the bomber can use technology the fighter is not using then the bomber can be faster. The B-9 was NOT faster than the P-26. It was faster than the biplane fighters. In part because the P-26 used a smaller, lighter engine that gave the same power as the engines in B-9, despite the fixed landing gear and wire bracing.
What happens to the B-9 if the opposing fighters use unbraced monoplane wings and retracting landing gear?
Granted the perspective is a bit off but which airplane needs more hours of design work (including blueprints) and more hours of construction to bring to the prototype stage?

You're correct that progress was moving quickly in the mid to late 1930s, but that progress was equally beneficial for large aircraft as for fighters. In any case, I think the main barrier to the pre-war development of an unarmed or minimally armed, fast heavy bomber was not technological but internal air force ideology, that being that the bomber will always get through without fighter escort; this assumption drove the design of US bombers. When the USAAF actually had to deal with an enemy with a powerful air force defending its cities, it found its bombers wouldn't get through without unsustainable losses (I think they also found that one or two raids wouldn't cause an enemy to sue for terms).

Designing a lightly armed or unarmed strategic bomber would require the concomitant design of a high-performance, long ranged escort fighter. (No doubt somebody will blame Congress, but before doing, they should remember that the USAAF leadership was politically well-connected enough so that Congress would generally follow the USAAF's priorities; the pot of money may not have been infinite, but the USAAF had a great deal of influence over how their share of that pot was distributed, and they didn't distribute it to long-ranged escort fighters) Here, I think that the Luftwaffe, with its flawed zerstorer concept was actually considering enemy fighters as a serious threat to its bombers, showed somewhat better planning than did the USAAF. The USN, which included fighters in its carrier aviation planning, was also considering enemy fighters were a problem -- those fighters were there, at least partly, to prevent enemy fighters from decimating attack aircraft -- also showed a greater level of foresight.

In reverse, the Navy example is flawed because the Navy aircraft were restricted in size and power. They had to fit on the carrier and they had to be single engine. If you are using the same engine in your fighter and in your attack aircraft (carrying 500-1000lb bomb or a torpedo) then the attack aircraft is going to be slower than the fighter and perhaps shorter ranged when carrying the strike load. All have to take off and land from a certain sized "runway".

The Army can use twin engine bombers (some foreign nations used three) and gain range/payload over single engine aircraft.
By the time you get to the Martin B-10

you have a plane at the end of 1933 that had a normal range of around 590 miles, a max range of 1240 miles and a ferry range of 1830 miles.
or would when it became the B-10B. In any case the ranges were beginning to exceed what single engine fighters could achieve and still keep their performance. With the Armies interest in the B-15 and B-17 the range of fighters was falling further behind what the bombers could do.
The Boeing 299 was credited with a range of 2040 miles while carrying 2573 lbs of bombs. This was in 1935 with 750hp P & W Hornet engines. No single engine fighter in existence or in the planning stages could come close to that range using the engines available or promised at the time.

 

[swampyankee]

View attachment 476243




In reverse, the Navy example is flawed because the Navy aircraft were restricted in size and power. They had to fit on the carrier and they had to be single engine. If you are using the same engine in your fighter and in your attack aircraft (carrying 500-1000lb bomb or a torpedo) then the attack aircraft is going to be slower than the fighter and perhaps shorter ranged when carrying the strike load. All have to take off and land from a certain sized "runway".

The Army can use twin engine bombers (some foreign nations used three) and gain range/payload over single engine aircraft.
By the time you get to the Martin B-10
View attachment 476244
you have a plane at the end of 1933 that had a normal range of around 590 miles, a max range of 1240 miles and a ferry range of 1830 miles.
or would when it became the B-10B. In any case the ranges were beginning to exceed what single engine fighters could achieve and still keep their performance. With the Armies interest in the B-15 and B-17 the range of fighters was falling further behind what the bombers could do.
The Boeing 299 was credited with a range of 2040 miles while carrying 2573 lbs of bombs. This was in 1935 with 750hp P & W Hornet engines. No single engine fighter in existence or in the planning stages could come close to that range using the engines available or promised at the time.

I disagree with your dismissal of the USN example; while the USN had restricted its operational carrier aircraft to have one engine*, the navy planners could have come to the conclusion that "the bomber will always get through" just as easily as could the USAAC or it could have decided that using fighters to defend carriers from air attack was pointless, and build heavily armored carriers instead. The basic specifications of the B-17 are interesting: here's an aircraft that is tasked with bombing somebody, but the most likely somebodies are Japan, clearly impossible to attack from US bases, and Europe, also impossible to attack from US bases. About the only countries that could be bombed from US bases would be Mexico and Canada. Was the B-17 really designed to attack foreign cities or was it really designed as a long-range adjunct to coastal artillery?



-------------

* The USN had operated twin-engined aircraft from carriers in the 1920s and early 1930s, the T2D. I suspect they found it to be feasible but not cost-effective.

 

[Shortround6]

They didn't really operate twin engine aircraft from carriers in a normal sense.
Douglas T2D.

Yes they were operated from the Langley but lowering them over the side and recovering by crane are not normal carrier operation.
Most were used from shore bases. With a 57ft wing span and 42 foot length their ability to operated from carriers was limited. No mention of wing folding and getting something that size down the elevators probably wasn't happening.

" About the only countries that could be bombed from US bases would be Mexico and Canada. Was the B-17 really designed to attack foreign cities or was it really designed as a long-range adjunct to coastal artillery?"

doesn't really matter from the point of view of an escort fighter. You simply couldn't build an escort fighter with the range required to stay with the B-17 no matter what target was intended. In fact building fighters to escort the B-18 was going to be difficult. With 882 gallons on board a B-18 could fly 1200 miles while ferry range was 2225 miles.

I would also note that the B-17 was sort of an interim bomber. It wasn't what the Army really wanted which is why they were building the B-15, the B-19 and funding other design studies of very large bombers. It use air cooled engines which the Army bought because they were available but the army was funding the "hyper" engine/s and the Allison as they thought they were better suited to turbo operation at high altitudes for long range.
If you can't build a fighter to escort your "interim" less than optimal bomber/s what chance do you have of building escort fighters for the really big bombers they were planning for the future?

 

[KiwiBiggles]

Was the B-17 really designed to attack foreign cities or was it really designed as a long-range adjunct to coastal artillery?

The clue is in the name. Early B-17s were not covered in guns; they were dubbed Fortresses as they were designed to act as a longer-ranged coastal defence fortress.

 

[swampyankee]

The clue is in the name. Early B-17s were not covered in guns; they were dubbed Fortresses as they were designed to act as a longer-ranged coastal defence fortress.

That disagrees with, among other sources, Boeing's web site: "Richard Smith dubbed the new plane, with its many machine-gun mounts, the 'Flying Fortress,' a name that Boeing quickly adopted and trademarked" (Boeing: Historical Snapshot: B-17 Flying Fortress)

On the other hand, the idea that the USAAC/USAAF bomber force was intended as long-range coastal defense makes a great deal of sense, in that the number of defending aircraft a bomber performing maritime attack would need to contend with would be fairly small.

 

[Shortround6]

The USAAF had "plan" that stretched over years if not over 1-2 decades. Much like the RAF and what would become Bomber Command. They wanted to become, at the least, fully the equal of the Navy and the ground army and at best, the premier military service in the Country. Much like the RAF ordering hundreds (or thousands) of Fairey Battles and Blenheims to build the RAF while they waited for better designs. The USAAF had to order interim aircraft to train crews, establish a position in the "budget wars" (get the congress or higher ups used to the idea of funding bombers) build bases,etc while they worked towards their "dream" aircraft, which would have intercontinental range.
The requirement that lead to the XB-15 called for a 5000 mile range.
It also resulted in the XB-16 design study

which morphed from 4 to six engines. The range requirement was restated as "maximum practical"
Boeing also came up with the XB-20 to compete with the XB-19 but as the XB-20 was pretty much a slightly bigger XB-15 with bigger engines it wasn't as advanced as the XB-19 and was not funded.
Please note that only the XB-16 proposal used liquid cooled engines and the Army was sticking to the "idea" that liquid cooled engines (buried in the wings) were the preferred power plant for their ultimate bomber.
There may have been a lot of paper doodles that never got an official designation.

In the political climate of the US in the 1930s, publicly announcing you wanted intercontinental bombers would have been career suicide so euphemisms or fuzzy mission statements were the order of the day.

There was"talk" of putting those flat engines into the wings of a "fighter", but think about that. SIngle engine fighter with flat fuselage? twin engine fighter?
There may be some drawing/s somewhere. Please remember that one of the "concept" drawings for the P-38 shows a plane with two engines in the fuselage driving props on the wings through extension shafts and right angle drives to eliminate nacelles on the wing.

It is hard to figure out exactly what they were thinking during the 30s but the movers and shakers in the USAAF were certainly dreaming of very long range bombers and put more money and effort into the idea than any other country. It just took until the B-29 to actually pay off. (and air to air refueling)

 

[FLYBOYJ]

The clue is in the name. Early B-17s were not covered in guns; they were dubbed Fortresses as they were designed to act as a longer-ranged coastal defence fortress.

Upon touring the Boeing plant and witnessing the first B-17s roll off the assembly line, "An awestruck reporter called the aircraft a 'Flying Fortress,' and the name stuck."

How Did The B-17 Get Its Name? « Dr Burt

 

[Shortround6]

The name date back longer than that. One magazine article calls the B-9 a "Flying Fortess" and it might not have been original then?

See first sentence of text. Any heavily armed bomber may have been referred to as a "flying fortress" at the time and perhaps reporters weren't any more creative then than they are now.