On April 10, 1941, two P-39Ds were ordered modified and flight tested under contract AC18373 as flying testbeds for the experimental Continental V-1430-1 supercharged inverted-Vee engine that was expected to deliver 2100 hp. These aircraft were assigned the designation XP-39E. The company designation was Model 23. A third machine was later added to the order as a nonflying static test example. Serials were 41-19501, 41-19502, and 41-71464. I am not sure whether these were newly-built aircraft, or modified P-39Ds that were assigned new serials.
However, the Continental engine was not yet ready when the XP-39E airframes were completed, and the 1325 hp Allison V-1710-47 engine was installed in its place. In pursuit of better high-altitude performance, the -47 engine was equipped with a two-stage supercharger and drove an Aeroproducts propeller.
The XP-39E bore the same armament as the P-39D but featured a new wing with square-cut tips. Wing span and gross area were increased to 35 feet 10 inches and 236 square feet. Contrary to some other sources, the wing was not a laminar flow wing. Each of the three examples tested different vertical tail surfaces--the first being conical, the second being cut-off square and rather short, and the third being rather similar to that of the P-51. The carburetor air intake was relocated and the wing-root radiator intakes were enlarged. The fuselage was lengthened by 1.75 feet to accommodate the longer -47 engine.
Empty and loaded weights were 6936 lbs and 8918 lbs respectively, making the XP-39E the heaviest of all Airacobra variants. During tests, a maximum speed of 386 mph at 21,680 feet was attained, which was much better high-altitude performance than other Airacobra variants. An altitude of 20,000 feet could be reached in 9.3 minutes.
About two weeks after its maiden flight, the first XP-39E crashed during spin tests on March 26, 1942. Test pilot Bob Stanley parachuted to safety.
The second aircraft flew on April 4, 1942. Following the loss of the first example, the static test example was brought up to flying condition.
The XP-39E had a much better high-altitude performance than other Airacobra variants. It was redesignated XP-76 and no less than 4000 were ordered by the USAAF. However, the new design was considered to be inferior to the basic Airacobra in many respects, and the order for the XP-76 was later cancelled in its entirety. Nevertheless, the XP-39E was to provide some valuable basic data for the later P-63 Kingcobra.
41-19501/19502 Bell XP-39E Airacobra 41-71464 Bell XP-39E Airacobra