A favorite with the Soviet Air Force, the P-63 was distinguished by and aft mounted engine that drove the propeller by a hollow 10 foot shaft through which a 37mm cannon could be fired.
P-63 began as improved P-39, redesigned around the new NACA laminar-flow wing, basic airfoil section used in P-51, and a new Continental engine intended to replace the Allison. Wings tested on thr3ee modified P- 39Ds resulting in a go-ahead for 2 prototype XP-63s ordered 27 Jun 41. Deliveries began Oct 43. Of 3,303 built, 2,421 went to USSR, 300 to Free French, and others were RP-63 "Flying Pinball Machines" target aircraft with heavy armor, fired at by gunners in training using frangible bullets; a light would flash on the aircraft when hit. Lacked range and altitude of contemporary fighters. Without ballast in the nose, or comparable weight of ammunition for cannon, center of gravity went too far aft and made handling difficult.