The P-39 could not compete with the Japanese Zero, but was used from Australia to the Alutians. The engine is behind the pilot, with a drive shaft that runs between his legs, to make room for a 37mm cannon in the nose. The Russians used them as tank busters.
Prototype ordered Oct 37 on basis of originality of its armament--a 37 mm cannon installed in the nose and firing through prop hub. Cannon installation dictated entire structure of aircraft resulting in engine installation in center of fuselage behind pilot, and a long propeller shaft routed between the pilot's legs. Had a completely retractable tricycle landing gear, first for a fighter aircraft. First prototype showed great qualities in both horizontal and vertical flight. However, a series of modns followed which transformed it from a thoroughbred into a draft horse. Most responsible for its lack of success as an interceptor (and lack of high altitude performance) was replacement of experimental supercharged engine, used in prototype, with unsupercharged engine. Worse, aircraft was now heavier with its armor plating and armament installed in accordance with original specifications. Thus modified, prototype suffered significant decline in performance. Nevertheless, Aug 39 aircraft ordered into mass production; first Cs delivered Jan 41. The following D model offered to RAF who rejected it; however, several hundred went to USSR and about 250 to USAAC Dec 41. Had good flying qualities and rugged airframe but low rate of climb, low service ceiling, and relative lack of maneuverability. Properly flown it became an important ground support aircraft with firepower to inflict heavy damage and capacity to absorb punishment. About half (4,773) of all - 39s built went to USSR. Although outclassed by contemporary enemy fighters, served well in the Pacific as a bomber escort, interceptor of Japanese air raids, and ground attacker. First all-black fighter group, the 332nd, equipped with "Q" models Jan 44 and used them to great advantage. Was quickly replaced in Europe as P-38s, -47s, and -51s became available.