P-36A

The Allison V-12, built by a division of General Motors, allowed a smaller frontal region than the radial engines which reduced drag. This engine was also used in the P-38, P-39, and P-51A. With the standardization of the Allison V-1710, the P-36 design was reworked to incorporate this engine, this resulted in three new designations of XP-37, YP-37, and XP-40.

The XP-37 which was equipped with a General Electric turbo-supercharger, and featured numerous other modifications, including a rearward positioned cockpit. The XP-37 showed at Army a top speed of 340 mph at 20,000 feet.
 

XP-37

Thirteen YP-37s were built for service evaluation, but differed little from the XP-37. The plane was lengthened 25 inches and the skin was Alclad which allowed a polished metal finish. Both the XP-37 and the YP-37 suffered from troubles with the turbo-supercharging system; the problems of supercharging the Allison was never fully solved until 1944.
 

YP-37

With increasingly ominous signs of an approaching war, development of this fighter was abandoned in favor of a less complex and more direct conversion of the P-36 for the Allison engine, the XP-40. With the promise from Allison that an internal supercharger could be fitted to the V-1710, the 10th P-36A was used. The armament would be the same as the P-36A; one .30 caliber Browning M-2 with 500 rounds on the top left of the engine cowling and one .50 caliber Browning M-2 with 200 rounds on the top right of the cowling. Provisions were made to carry six 20 lb bombs. Fuel capacity would be 158 U.S. gallons and since the prototype would consist largely of a preexisting design, the cost was a real bargain. Although from the firewall back the P-36 and XP-40 look virtually the same, much more was considered than just mounting the Allison on the P-36. The liquid-cooled Allison required many systems (coolant radiators and tanks) not needed by an air-cooled engine. These along with gun mountings, air induction systems, weight and balance and maintenance considerations had to be woven into the new design.

Because the 1150 HP V-1710-19 did not have a turbo and an intercooler (this affected high altitude performance), the radiator was mounted externally far aft below and behind the cockpit (similar to the later North American P-51).

The initial XP-40

The XP-40 made its first flight on October 14, 1938 and was accepted by the Army on the 16th. The XP-40 showed at Army a top speed of 342 mph at 11,000 feet. Although there had been wind tunnel tests, air flow and cooling problems showed up in flight tests. Various fixes were tried which lead to the radiator being moved forward under the engine.

Final version of the XP-40

In January 1939 the XP-40 competed against the Bell XP-39 and the twin-engine Lockheed XP-38 with turbos (it crashed after continuing its delivery flight past Wright Field to set a new coast-to-coast speed record). The XP-40 won the competition and the Army immediately ordered 524 production models (the army ordered later ordered models of both the P-38 and P-39). The first production P-40 was identical to the XP-40 except for the armament.

The P-40 experienced lateral control problems. One attempt to fix this problem was the addition of a "dorsal fin". The above is an early production P-40F. The fin didn't work well and eventually the fuselage was lengthened to correct the problem. A similar "fin" was later attempted on late P-40Es and all P-40K models, but again it was abandoned in favor of a lengthened fuselage.

A twin-engined design made it to the mock-up stage. This aircraft seems to be P-40C S/N 41-13456, but the canopy seems to be from a P-40D and the engines are P-40F type Merlins. No record of this example has been found.

This is a picture of the XP-40F. It was an attempt to re-steamline the fuselage of the P-40. The size of the intake region had grown as the models progressed. This was the third production P-40F with the intake moved aft. Whatever results followed did not lead to any design change.

The last major modifications began with the XP-40N; an unofficial unidentified production P-40N used for test work. A one piece bubble canopy was installed and the turtledeck was cutdown.
 

XP-40N

Three converted P-40s became the XP-40Q series; a cleanup and lightening program that began in 1943. The first, XP-40Q-1, was a converted P-40K with a 1425 hp V-1710-121 Allison with a two-stage supercharger and a four bladed prop for increased altitude performance. The coolant area was moved from the chin to the wing roots, armament reduce to 4 .50-calibers, and no bomb provisions were added.
 

XP-40Q-1

The XP-40Q-2 was similar to the XP-40N; it was fitted with a two piece bubble canopy and the turtledeck was cut down. The chin coolant area was trimmed down and the oil cool was carried in the wing. The wing were clipped by nearly two feet and the tips squared off. The result of the modifications was a performance of 422 mph at 20,500 feet (the fastest of the P-40s). This plane eventually found a civilian owner and was used as a racer (eventually crashed).
 

XP-40Q-2

The final P-40 was the XP-40Q-3. This plane was converted from a P-40N. It was similar to the XP-40Q-2; it had clipped wings and a slightly different, three-piece, bubble canopy.
 

XP-40Q-3

By the time these experimental P-40s reached the performance listed, early in 1944, there was no longer a valid reason for the military to produce an updated P-40 and the program was dropped. At the end of the war the P-40 had almost been entirely replaced as a Army combat model. Only one group, 23rd Fighter Group (formed from the AVG), still flew the plane.

Final Notes

The P-40 is misunderstood and does not get historical accolades and considerations like the P-38 or the P-51. Many historians and revisionist hover on the facts that the P-40 was nearly obsolete before it was delivered and had several limitations. YES, the P-40 was developed from an older design. YES, it had altitude limitations, but it was a tough plane that had a good roll rate, good armament, dive speed, and could take punishment. Imagine if the plane had received the turbo-supercharger originally considered!

When pilots were trained to use the P-40's strengths to exploit the enemies weaknesses, the plane was a formidable foe. How can the historians forget that the P-40 was the plane used to amass the greatist military kill/loss ratio ever?

Erik Shilling (AVG Pilot), "The P-40 was a hell of a lot better fighter than those who have never flow it think. If it had had the top speed of a 51 I would take it over any fighter the US had."

Jeff Ethell (Pilot, aviation author and historian), "After years of reading that the P-40 could not maneuver, particularly with a Zero, and that it had to make diving slash attacks to be effective, I had come to accept the general opinion that it was outclassed by almost everything else flying. Sitting in the cockpit, with the controls in my hands, having written a book about the aircraft and said all those things, the accepted history in my brain was wrestling with the seat of my pants. No question it did not have the top speed and high altitude performance to disengage targets at will, but it was certainly more maneuverable than other American fighters, particularly the P-51."