The first P-39D Airacobras entered service with the USAAC in February 1941, first with the 31th Pursuit Group (39th, 40th, and 41st Pursuit Squadrons) based at Selfridge Field, Michigan.
At the time of Pearl Harbor, the USAAAF had five pursuit groups flying the P-39 Airacobra. These were the 8th Pursuit Group based at Mitchell Field near New York City, 31st and 52nd Pursuit group at Selfridge Field, Michigan, the 36th Pursuit Group based in Puerto Rico, and the 53rd based at MacDill Field in Florida. I also have a reference which lists the 16th Pursuit Group based in the Canal Zone,and the 31st Pursuit Group based at Baer Field, Indiana. The 15th Pursuit Group's 47th Pursuit Squadron became active at Wheeler Field in Hawaii on December 21, 1941.
At the time of Pearl Harbor, the P-39 (along with the P-40 and a few P-38s) was virtually the only modern fighter available to the USAAC. Those P-39s already in service with the USAAF at the time of Pearl Harbor were deployed at home bases, but were quickly moved forward to overseas bases in Australia, Alaska, Hawaii, Panama, and New Guinea to try and stem the Japanese advance.
On April 30, 1942, thirteen P-39s from the 35th and 36th Pursuit Squadron flew their first combat mission under Lt Col Boyd D. Wagner. For the next 18 months, the P-39 and the P-40 were the principal front line equipment of USAAF fighter units in the Pacific. They carried much of the load in the initial Allied efforts to stem the rapid Japanese advance. Many Allied pilots lacked adequate training, and equipment and maintenance were below average. The Airacobras operating in the Southwest Pacific were sometimes called upon to serve as interceptors, a role for which they were totally unsuited. They proved to be no match for the Japanese Zero in air-to-air combat. In fact, because of difficulties with the oxygen supply, the Airacobra was not even able to reach the Mitsubishi G4M (code name *Betty*) bombers raiding from altitudes above 25,000 feet. In the laconic words of the official AAF history: "The Airacobra, even in a good state of repair, was unable to meet the Japanese fighters on equal terms." Experienced Japanese pilots such as Saburo Sakai regarded the Airacobra as a relatively easy "kill". The P-39s were not as manauverable as the lighter and more nimble Japanese fighters, and enemy fighters could often avoid combat with the P-39s by outclimbing them. Nevertheless, the Airacobra was quite tough and was able to absorbing a great deal of battle damage and still keep on flying, and its armament was able to deliver lethal blows to many a lightly-armored Zero.
Some interesting hybrids were produced during those days. The 67th Fighter Squadron was responsible for fitting a P-39D wing to a P-400, and a little later the 68th Fighter Squadron produced a P-400 fuselage with one P-39D wing, one P-39K wing, and an Allison V-1710-63.
The 31st Fighter Group was provided with Airacobras in Southern England in August of 1942. Between August and October of 1942, the Group participated in missions against enemy targets in France. The Group suffered heavy losses in air-to-air combat against the Luftwaffe, and the 31st FG re-equipped with Spitfire Mk Vs.
With the formation of the US Twelfth Air Force in the Middle East in the Autumn of 1942, Airacobras saw service in the Mediterranean area with the 81st and 350th Fighter Groups and two squadrons of the 68th Observation Group. These aircraft were diverted from a Soviet consignment, being a mixture of P-400s and P-39D-1s. In the Middle East, the Airacobras were used primarily for very low-altitude strafing missions, escorted by Warhawks or Spitfires. They took part in the Allied landings in Tunisia, at Anzio, in Sicily, and operated throughout the entire Italian campaign. In spite of the Airacobra's obvious deficiencies, units using the P-39 achieved the lowest loss rate per sortie of any USAAF fighter used in the European theatre.
Airacobras based in Alaska took part in the battles for the Aleutians. On September 14, 1942, Airacobras of the 54th Fighter Group took part in the first counter-strike actions against Japanese forces in the area. Airacobras were also deployed to the Canal Zone to defend the Panama Canal, but no action ever took place there.
The Airacobra reached its peak usage in the USAAF in early 1944, with over 2100 in service. However, the drawdown was fairly rapid after this, as they were quickly replaced by P-38s, P-47s and P-51s. By April of 1944, the last P-39 squadrons in New Guinea (the 82nd and 110th Tactical Reconnaissance Squadrons) had turned in their Airacobras for other aircraft. The 347th Fighter Group was the last to fly the Airacobra in the Southwest Pacific in August of 1944 before re-equipping with P-38s. Thereafter, P-39Qs were flown at training bases in the United States until the end of the war.
The following Fighter Groups operated the P-39 between 1941 and 1945, but in some cases only relatively briefly:
P-39s were also used by the 342nd Composite Group (33rd Squadron) and the 59th Observation Group (488th, 489th and 490th Squadrons). The P-39 was also used by the 48th, 84th, 85th, 339th, 494th, 405th, 496th, and 478th Bombardment Groups in the training role.