Why didn't the army develop the P-51?

The USAAF, apparently awaiting the arrival of the P-38, lacked a decent high altitude interceptor at the start of WW2. As a stop-gap measure, it deployed large quantities of Allison 1710-39 powered (single-stage supercharger) P-40Es in that role. Yet the P-51, powered by the same engine, had been flown and shown to be clearly superior to the P-40E, especially in those attributes necessary for an interceptor. The RAF had two very effective high altitude interceptors in its RR Merlin powered Hurricane and Spitfire and evidently wanted to bolster its ground attack capability by buying large quantities of the export variants of the P-40E and P-51 performance optimized for intermediate altitude operations.

Starting in December 1941, the IJN launched the first in a continuous series of high altitude (above 27,000 feet) raids on Southeast Asian allied installations culminating in attacks on Darwin. Attempts by P-40E fighters to intercept these many attacks were essentially failures. I do not believe there was one successful interception during the first four months of the Pacific war and relatively few until the advent of the Guadalcanal campaign where large numbers of F4F-4 were deployed.

Considering how desperate was the situation in SE asia in early 1942 and the USAAF's lack of a fighter with performance on a par with contemporary types, why wasn't the USAAF more proactive in developing and fielding the P-51?

It is my understanding the P-51 was developed for the RAF because the US Army wouldn't give up any of it's P-40 production to what the RAF was requesting. North American came in and said they could build a better plane for them. The USAAF stuck a couple of them in a hangar, just to be arse holes. Until someone had a Penny Drop and "what about them British thingies"?

In a nut shell. I may be mistaken.

I've wondered myself. The single-stage ALlison wasn't bad. It was developed as stipulated and the Army took away the turbo from all but P-38 use. The Bf 109 also has a single stage supercharger and flew quite nicely at high altitudes. So my wonder for years has been why the Army didn't ask Allison to develop the slngle-stage supercharger into a variable hydraulic drive unit?

If they had, then the Allison would have been aperfect complement to the Merlin and maybe the P-51 would never have needed the Merlin to start with. Allison asked the Army if they wantred a 2-stage supercharger on at least two occasions and was told "no" both times.

That being said, I have often wondered why the P-51 wasn't of more interest to the US military until later myself. It is somewhat baffling. Of course, so is the Bell Airacuda ... if they could order THAT, maybe we KNOW why the Army wasn't interested ina hgih-performance aircraft ... they were stuck in 1930's art deco mode rather than pragmatic analysis of what was needed.

The Army was also anticipating the P-39, but with only the single-stage instead of the two-stage turbo, it sucked at anything over 20,000 feet. No one really knows why Bell nixed the two-stage setup, because the NACA parasitic drag tests showed that the cooler intake was not a major factor...

N4521U said:

It is my understanding the P-51 was developed for the RAF because the US Army wouldn't give up any of it's P-40 production to what the RAF was requesting. North American came in and said they could build a better plane for them. The USAAF stuck a couple of them in a hangar, just to be arse holes. Until someone had a Penny Drop and "what about them British thingies"? In a nut shell. I may be mistaken.

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According to P-40 by Ray Wagner - Page 2, by the end of 1940, the RAF had received 558 Tomahawk I, IA and IB aircraft. AHT seems to indicate that by the end of 1941, deliveries of P-40 exports exceeded 1,000 units. Like Oliver, the BPC was simply interested in more please of the type than american industry was yet able to produce. I have no evidence, but it seems logical that the USAAF was at least a silent party to the BPC discussions with North American. In any event, once a working model of the P-51 can be seen by the USAAF and the two aircraft can be compared, it would seem to be a no-brainer to develop the one over the other. What forces prevented that from happening? I smell some sort of political interference, but then, I am chronically paranoid.

separate question: Seems like a single-stage, two-speed supercharger might have been a simpler and quicker solution to the Allison 1710's high altitude anemia. that seemed to work reasonably well for the early USN radials. Was the Bf-109's single-stage SC one or two speed unit?

Why wasn't U.S. Army more proactive in developing a decent 20mm aircraft cannon?
Why did Britain field an "Infantry Tank" armed only with a single machinegun plus a 2 pounder cannon which lacked HE ammunition?
Why didn't German Navy resume their WWI era guided weapons program during 1935 when they had the money to do so?
Why did 1930s Italy pour a huge amount of money into East Africa, a region of little military and no economic value?
Why did 1930s Japan pour a huge amount of money into Yamato class battleships when their army was short of almost everything and locked in combat with China?
Why did 1936 France wreck tank and aircraft production by nationalizing factories while simultaneously trying to conduct a huge military expansion?

WWII era had many world leaders who were boneheads. Just like today.

The 109 used a variable speed drive, much like the torque converter from an automatic transmission. A low altitude it "slipped" and as altitude was gained it progressively tighten up until at it's critical altitude (varied depending on model of engine) there was little or no slippage. Impeller speed varied from a little over 7 times crankshaft to about 10 times crankshafts speed.

The DB engines big altitude advantage was that it didn't use much boost to begin with. Rounding things off 1.3 AtA is about 39in MAP or about 4 1/2 lbs boost and 1.42 AtA is about 42.6in MAP or about 6 1/2lbs boost.

Allisons ran about 44 in for military power (1.46 AtA) and could run up to 57in WEP on some models for 1.9 AtA.

The problem comes in with the facts that at about 18,000ft the air pressure is about 1/2 what is is at sea level and at 22,000ft a cubic ft of air is about 1/2 the weight of a cubic ft at sea level.

This means the supercharger has to compress the outside air 2.94 times for the Allison for "military" power (1150 hp ?) and 3.81 times for the 57 in WEP setting ( other WEP settings differ).

The DB engine needs to compress the air 2.61 times for 1.3 AtA and 2.85 times for the 1.42 AtA.

You can only spin the impeller so fast, once the tip speeds go supersonic they set up shock waves inside the supercharger that totally disrupt air flow. At the beginning of the war nobody had a single stage supercharger that went to a pressure ratio of 3 to 1 with any sort of decent efficiency. Poor efficiency not only takes more power to drive but the extra power simply heats the intake charge to a higher temperature make the charge less dense (lower power) and brings it closer to the detonation limit ( less boost can actually be used). Also please note that nobody had a single stage centrifugal compressor (supercharger or jet engine) of over 4 to 1 installed in an aircraft at the end of WW II ( test bench rigs may do a bit better.)

Most two speed supercharger were used to boost take-off and low altitude power with the low gear rather than boost altitude performance. The American radials ( and the Bristol Pegasus) having critical altitudes in high gear lower than an Allison using the 9.60 gear ratio.

Allisons that used 9.60 supercharger gears were about as good as it was going to get. The 8.80 gears used in many of them were a bit of a compromise.

In any event, once a working model of the P-51 can be seen by the USAAF and the two aircraft can be compared, it would seem to be a no-brainer to develop the one over the other. What forces prevented that from happening? I smell some sort of political interference, but then, I am chronically paranoid.

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Hey Oldcrow, there's been similar discussions on the forum about American involvement in the development of the Mustang in the past. Just a couple of points worth noting. I think its a matter of timing as much as anything, perhaps even reticence. In early mid 1941 the United States was not yet at war and therefore had no real experience with the P-40 in combat against a modern enemy such as the Luftwaffe - apart from what they had heard from the Brits. The British were asking for P-40s and the Mustang came about as a result of Curtiss being unable to fulfil their request. The first production RAF Mustang first flew in April 1941 and the Brits didn't get their hands on it to evaluate it until later that year. Mustangs didn't start arriving in the UK until late 1941 and began entering service in January 1942.

From what I can recall off the top of my head, part of the conditions behind an American firm building a military aircraft for the British in wartime was that two examples of the aircraft were to go to the USAAC for evaluation; it was one of these that was eventually modified with a Merlin to become the USA's first XP-51B in mid late 1942 - by which time the USA was at war. There might have been political reasons why little attention was paid to the Mustang in 1941, but it might also be because there was no real pressure for a new fighter as the USA wasn't at war; once the declaration of war came about things began moving pretty quickly for the Mustang/P-51.

Having had discussions with Bill (Drgondog) on the forum, he asserts that its likely that the Americans, either NAA and/or Packard or maybe the USAAC/F showed interest in fitting a Packard V-1650-1 (Merlin 28 ) to the Mustang prior to the British suggestion of a Merlin Mustang in April 1942. Curtiss had began experimenting with the P-60 powered by a Merlin by that time - as an aside. When the RAF team that first test flew the RAF Mustangs in the USA before they were shipped to the UK the Brits were impressed and did comment on the Mustang's lack of altitude performance even at that time. From early trials it was clear that the Mustang had better performance than the P-40 and although tacit interest might have been paid to the Mustang by the USAAC/F, for whatever reason it was following British experience with the aircraft and British enthusiasm for a Merlin engined Mustang (and a subsequent declaration of war) that the Americans really got into the Mustang/P-51.

Once thing is for certain is that the Americans were in favour of a Merlin powered Mustang from the first correspondence between the British and the Americans about the idea (April/May 1942). Certainly the suggestion of placing a Merlin 28 in Mustangs interested both sides, the British more as an expediency and so as not to disrupt the flow of Merlin 61s to Spitfire IXs, but no Mustangs were so converted. The British investigated both the two-speed two-stage Merlin and the 'XX and found a big leap in performance with the 60 Series engine, so proceeded with that instead, and although American interest within NAA and Packard was high, action within the USAAF tended to go a little slower.

Here's an extract from a letter sent to Jimmy Ellor with Packard in Detroit by Lappin of RR dated 8 September 1942:

"Here we have an aircraft, which from the point of view of aircraft, is outstanding, and even with a Packard 28 it would have a performance probably around 400 mph. However, I suppose it is one of those situations where in America you do not know where to start in order to get a large number of people with divergent views to see the common sense policy. However, we are taking energetic action as far as possible in order to impress upon the Air Ministry here and the American Air Corps that this machine should enjoy a very high priority, which at the moment, it has not got."

Good info from SR6 and Nuuumannn.

My main interest in this thread is the ancient history of the Allison powered P-51, separate from its evolution into the Merlin incarnation. Especially the Mustang I/P-51.

relevant AHT Chronology:

Jan '40. In response to BPC request to license build P-40 A/C, NAA proposes to build a fighter A/C superior to the P-40.

April 24, 1940, NAA Proposal for fighter design NA-73X is accepted by BPC.

May 4, 1940, BPC approves NA-73X preliminary design.

May 1940, USAAC releases NA-73 for UK sale but requires deliveries of two A/C for testing. (The two production NA-73 to be delivered will be designated XP-51.)

May 29, 1940, contract with USAAC for two production NA-73 is signed.

October 26, 1940 first NA-73X flight.

Nov 20, 1940, NA-73X destroyed in crash on 9th flight. Late Entry: (other sources say A/C was repaired and flight tests continued)

April 25, 1941, First production NA-73 is flown. AG345

May 20, 1941 First flight of AG348 (USAAC XP-51), 5th NA-73 (including NA-73X) Mustang I.

Aug 24, 1941, XP-51 delivered to Wright Field, Ohio.

Dec 16, 1941, second XP-51 delivered to Wright Field.

Jan, 1942, NA-73 Mustang I tested at Boscombe Downs.

I assumed the USAAC had 6 months of extensive NA-73X flight test data upon which to make a judgement. Evidently, like the early P-38 crash, this couldn't happen foregoing underlined replaced by may not have happened. So the clock on USAAC awareness of NA-73 performance may really only start ticking in April '41 at the first flight of the production NA-73 Mustang I.

It would be interesting to know if the company flight test data from the 9 flights in 1940 was extensive enough to clearly show superiority over the P-40.

I am curious, though...at what point did they realize a need for a high-altitude performer?

Up until 1941, no serious combat was performed at altitudes over 20,000 ft. except by the Japanese. If I remember correctly, even the Battle of Britain was fought on average at 15,000 feet.

GrauGeist said:

I am curious, though...at what point did they realize a need for a high-altitude performer?

Up until 1941, no serious combat was performed at altitudes over 20,000 ft. except by the Japanese. If I remember correctly, even the Battle of Britain was fought on average at 15,000 feet.

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That might have been an average but combatagainst 109s was anything from 30,000ft downwards.

In part this was dictated by Luftwaffe tactics, with fighters always trying to gain the 'high ground'. This trend lasted through 41 (109F vs Spit V), only when the 190 A series came along did the altitudes come down. Basically because the 190As were crap at high altitudes and the RAF was 'offering combat', therefore it followed the 190s down. You can argue as to the logic of it all but even the most UK biased observer can question the RAF's tactics in 41 and 42 (then again this was Leigh Mallory's time where he seemed determined to race Bomber Command to see how many planes he could lose).

In North Africa and Malta altitudes came down again, but the 109s with their far superior altitude ability basically slaughtered the Hurricanes and P-40s of the time, as they could pick where and when they wanted combat.

Altitudes went up again with the USAAF bombing campaign, with 20,000+ft being common (and hence the need for the 190D as the 190As didn't have the altitude performance).

The Mustang Is, if they had been in North Africa would have done better than the P-40s and Hurricanes of course, but they would have still been hammed by the 109Fs and Gs. With only low altitude performance you are a victim waiting to be hit. No matter how fast you are a (say) 109 with 10,000ft advantage means it can pick and choose when to hit you.

In Northern Europe, with its regular bad weather and low clouds the Mustang Is (P-51A) did sterling service at low level fighter recon, with low cloud it didn't help a 109 or 190 to be 10,000ft above as it wouldn't have been able to see you. In fact the 2nd TAF used them right up until they had none left in late '44.

But (say) the 51As had been more available and the RAF sent them in at 25,000ft against 109s over France in 41, the Luftwaffe would have shot them out of the air in droves.

Possibly a better question might be why Mustang production was not augmented by Allison engined low level A36s? Especially if the USA version of the 20mm had received the simple solutions for reliability. Dive brakes and everything.

GrauGeist said:

I am curious, though...at what point did they realize a need for a high-altitude performer?

Up until 1941, no serious combat was performed at altitudes over 20,000 ft. except by the Japanese. If I remember correctly, even the Battle of Britain was fought on average at 15,000 feet.

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I didn't realize that, I assumed the entire BoB was fought at altitudes of ~30,000' I also thought that, in general, the motivation for the USAAC development of the P-38 was the perceived need for a high-altitude interceptor which would place realization pre-1940, wouldn't it? Cherished Assumptions falling like trees here.

OldSkeptic said:

That might have been an average but combat against 109s was anything from 30,000ft downwards.

In part this was dictated by Luftwaffe tactics, with fighters always trying to gain the 'high ground'.


Altitudes went up again with the USAAF bombing campaign, with 20,000+ft being common (and hence the need for the 190D as the 190As didn't have the altitude performance).

The Mustang Is, if they had been in North Africa would have done better than the P-40s and Hurricanes of course, but they would have still been hammed by the 109Fs and Gs. With only low altitude performance you are a victim waiting to be hit. No matter how fast you are a (say) 109 with 10,000ft advantage means it can pick and choose when to hit you.

In Northern Europe, with its regular bad weather and low clouds the Mustang Is (P-51A) did sterling service at low level fighter recon, with low cloud it didn't help a 109 or 190 to be 10,000ft above as it wouldn't have been able to see you. In fact the 2nd TAF used them right up until they had none left in late '44.

But (say) the 51As had been more available and the RAF sent them in at 25,000ft against 109s over France in 41, the Luftwaffe would have shot them out of the air in droves.

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Good comments but where were the bombers flying? I am not talking F vs F here but rather F vs B. (interception) If the Bombers were flying above 25,000 ft and the escorts taking position above them during the Spanish CIvil war (As did the IJN later), that would explain much.

I was strictly speaking of the USA-prewar (7/12/41) period and the post PH 6 months when the unmet need for HIgh altitude performance seemed to peak for the the USAAF in the PTO.

Point of order Mr. Chairman; from AHT:

XP-51 = Mustang I (1st produced in April 41), produced in substantial numbers through April 1942.
P-51 = Mustang IA (production starts ~ July 42),
P-51A = Mustang II (production starts ~ March '43).

I am only considering Mustang I or XP-51. (My mistake above assuming Mustang I = P-51)

yulzari said:

Possibly a better question might be why Mustang production was not augmented by Allison engined low level A36s? Especially if the USA version of the 20mm had received the simple solutions for reliability. Dive brakes and everything.

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FlyboyJ posted elsewhere (http://www.ww2aircraft.net/forum/aviation/could-p51a-been-made-available-battle-midway-37586-3.html) that development of the A-36 was the result of a fungible money situation. No money for fighters in the budget but money for dive bombers lead to an initial order to expand NAA production capability.

In any event:

A-36, First flight Sep., 1942.

Would be nice to know the results of NAA testing of the first airframe (NA-73X) and subsequent tests of the Mustang I (or XP-51)

According to http://www.wwiiaircraftperformance.org/mustang/mustang-I.html

XP-51 (Mustang I) armed with 4 x 0.5" BMG and 4 x 0.3" RCMG (NAA report design ESTIMATES from April 24, 1940)
could make ~380 mph at ~13k' and
climb to 20k' in just under 9 minutes.
Mid-Latitude Ceiling ~31 to 32k'

This compares to the P-40E (same web source) armed with 6 x 0.5" HMG (USAAF report on TRIALS dated Sep 24, 1941)
could make ~340 mph at ~13k'
climb to 20k' in just under 12 minutes.
Mid-Latitude Ceiling ~30 to 31k'

Hi Shortround,

Love the discussion above in post #7! I get slightly different numbers when I run them, but it is essentially correct.

If I round off a bit, I get 1.3 ata as 37.65 inches MAP and 3.79 psi boost. I show 4.42 ata as 41.12 inches MAP and 5.50 psi boost.

When Allisons ran 44 inches MAP, it was 1.52 ata or 6.92 psi boost. 57 inches MAP is 1.97 ata or 13.30 pai boost. Actually wartime Allisons ran up to 75 inches MAP in combat according to pilots who flew them and that equates to 2.59 ata or 22.14 psi boost. They said a P-40 at 75 inches was a different animal from one at 57 inches. Wasn't exactly "by the book," but has been repeatedly reported.

Don't want to nitpick Shortround, nice post and pretty good info.

I'd post a spreadsheet that does all the conversions but I can't upoad a .xls or .xlsx file in this forum. I DID try and wound up posting it in another forum.

If this forum decides to allow us to upload Excel files, I'll post it here, too. Makes these discussions much easier.

B-17 bomber was designed to operate at high altitude from late 1930s. So was P-38 fighter aircraft.

P-51 wouldn't have been necessary if P-38 has performed as intended after a normal 3 year development cycle. That's the real problem. Fix P-38 early on or else replace it with P-51 program during 1941.

Most of the "fixes" were not able to be recognized until the P-38 got to Europe.

Two of the "fixes" thath COULD haven been addressed were the intake manifold issue and the poor cockpit heater.

Training the guys to fly a complex twin SHOULD have been seen as necessaary but maybe that would not have been recognized until it swam up and slapped them oin the face, and the differences between European and US gasoline probably would not have been unearthed until they were deployed. To me, the issue was that the people trying to correct problems were shoved aside in the rush to deploy and I'm not sure how to change that attitude unless the people trying to correct the iussues were somehow in power. That doesn't usually happen in real life.

davebender said:

B-17 bomber was designed to operate at high altitude from late 1930s. So was P-38 fighter aircraft.

P-51 wouldn't have been necessary if P-38 has performed as intended after a normal 3 year development cycle. That's the real problem. Fix P-38 early on or else replace it with P-51 program during 1941.

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The P-38 was designed to a 1937 specification for a high altitude interceptor. By 1943 the requirement changed. It wasn't a matter of fixing the aircraft, it was a matter of fitting into new requirements. It worked well, it did need improvement but there was nothing to fix on the P-38. Same could be said about the P-47 and yes, even the P-51.

The real fix "should have" been the AAF giving better multi-engine (twin) training to early P-38 pilots.

That doesn't speak well for U.S. Army Air Corps operational testing and training.

davebender said:

That doesn't speak well for U.S. Army Air Corps operational testing and training.

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It doesn't - but then in 1937 were they able to predict what was going to happen in 1943?