One of the persistent criticisms of the Bell P-39 Airacobra was that its high-altitude performance was rather poor in comparison to that of its competitors. The Bell P-76 was a proposed advanced version of the Airacobra that would be equipped with a more powerful engine, a laminar flow wing, and other refinements that would help to correct some of these deficiencies.
The P-76 project began its life as the XP-39E, which was a 1941 project to flight-test the experimental Continental I-1430-1 inverted-Vee supercharged liquid cooled engine. The company designation for this project was Model 23. The first two machines were ordered on April 10, 1941, with a third machine being added on October 17, 1941. As called for in the contract, three production P-39Ds were taken off the line and modified to XP-39E standards. Serials were 41-19501, 41-19502, and 42-7164.
The Continental engine was not yet ready when the XP-39E airframes were completed, and the 1325 hp Allison V-1710-47 engine had to be installed in its place. The carburetor air intake was relocated and the wing-root radiator intakes were enlarged. The XP-39E featured laminar-flow wings with square-cut tips, the wing span and gross area being increased to 35 feet 10 inches and 236 square feet respectively. Each of the three examples of the XP-39E that were built tested different configurations for the vertical tail surfaces--the first being rounded and tapered, the second being cut-off square and rather short, and the third being rather similar to that of the P-51. Armament was the same as that of the P-39D.
Empty and loaded weights of the XP-39E were 6936 lbs and 8918 lbs respectively. During tests, a maximum speed of 386 mph at 21,680 feet was attained. An altitude of 20,000 feet could be reached in 9.3 minutes.
The XP-39E was considered by the Army as being sufficiently different from the stock P-39 that it was considered worthy of a new pursuit designation--it was redesignated XP-76 on February 24, 1942. The Army felt sufficiently confident in the new aircraft that they ordered no less than 4000 production P-76s from Bell, all of which were to be manufactured at Bell's new plant in Atlanta, Georgia. Less than three months later, the USAAF was to have second thoughts about the wisdom of its decision. Even though the XP-76 had proven itself to be faster than the P-39D, the new design was considered to be inferior to the basic Airacobra in other respects, and the order for the P-76 was cancelled in its entirety on May 20, 1942, freeing up the Bell-Atlanta plant for license manufacture of the B-29 Superfortress.
Even though no production ensued, the XP-39E/XP-76 did provide some valuable basic data which proved useful for the P-63 Kingcobra project.