The first version to be supplied in quantity to the Soviet Union was the P-63A-7 with a higher vertical tail, and reinforced wings and fuselage. The fuselage proved to need strengthening, consequently in October 1944, a reinforcement kit for operational P-63s was developed.
Air Transport Command ferry pilots, including U.S. women pilots of the WASP program, picked up the planes at the Bell factory at Niagara Falls, New York, and flew them to Great Falls, Montana and then onward via the Northwest Staging Route through Canada to Alaska, where Soviet ferry pilots, many of them women, would take delivery of the aircraft at Nome  and fly them to the Soviet Union over the Bering Strait via the Alaska-Siberia route (ALSIB). A total of 2,397 (2,672, according to other sources) such aircraft were delivered to USSR, out of the overall 3,303 production aircraft. Sufficient aircraft continued in use after the war for them to be given the NATO reporting name of Fred.
Model Scale 1:38
Soviet P-63 "Fred"
L-39 Swept Wing Testbed