Bell P-63 Kingcobra
While the P-39 had originally been introduced as an interceptor, later in its development it was decided to reduce the cost and complexity of the engine by removing the turbocharger. High-altitude performance suffered dramatically as a result, and Bell proposed an experimental series to test out a variety of solutions.
The XP-39E proved to be faster than the standard Airacobra; a maximum speed of 386 mph being attained at 21,680 ft during tests. However, the XP-39E was considered to be inferior to the stock P-39 Airacobra in all other respects, so it was not ordered into production.
Although the XP-39E proved to be disappointing, the USAAF was nevertheless interested in an even larger aircraft based on the same basic layout. Even before its first flight, the USAAF placed an order on 27 June 1941 for two prototypes of an enlarged version powered by the same V-1710-47 engine. The new design was given the designation XP-63 and serials were 41-19511 and 41-19512. A third prototype was also ordered, 42-78015, this one featuring the Packard V-1650, the U.S.-built version of the Rolls-Royce Merlin engine. In September 1942, even before the prototype flew, the USAAF ordered it into production as the P-63A (Model 33). The P-63A's armament was to be the same as that of the then-current P-39Q, a single 37 mm M4 cannon firing through the propeller hub, two synchronized 0.50 in machine guns in the upper nose firing through the prop, and two 0.50 in machine guns in underwing gondolas. The trajectory of the 0.50 in guns was far flatter than that of the cannon. Its main use in American service was the unusual one of a manned flying target for gunnery practice. The aircraft was generally painted bright orange to increase its visibility. All armament and the regular armor was removed from these RP-63 aircraft, and over a ton of armored sheet metal was applied to the aircraft. This was fitted with sensors that would detect hits, and these hits were signaled by illuminating a light in the propeller hub where the cannon would have been. This earned the aircraft the unofficial nickname of Pinball. Special frangible rounds made of a lead/Bakelite combination were developed that would disintegrate upon impact. These were known as the "Cartridge, Caliber .30, Frangible, Ball, M22".