The United States requisitioned 200 of the aircraft being manufactured for the UK, adopting them as P-400s (named for the advertised top speed of 400 mph (644 km/h)).
After the attack on Pearl Harbor, the P-400 was deployed to training units, but some saw combat in the Southwest Pacific including with the Cactus Air Force in the Battle of Guadalcanal.
Though outclassed by Japanese fighter aircraft, it performed well in strafing and bombing runs, often proving deadly in ground attacks on Japanese forces trying to retake Henderson Field.
Guns salvaged from P-39s were sometimes fitted to Navy PT boats to increase firepower.
Pacific pilots often complained about problems of performance and unreliable armament, but by the end of 1942, the P-39 units of the Fifth Air Force had claimed about 80 Japanese aircraft, with a similar number of P-39s lost.
Fifth and Thirteenth Air Force P-39s did not score more aerial victories in the Solomons due to the aircraft's limited range and poor high altitude performance.

Airacobras first fought Japanese Zeros on 30 April 1942 in a low level action near Lae, New Guinea.
From May to August 1942 combat between Airacobras and Zeros took place on a regular basis over New Guinea.
 Compilation of combat reports indicates the Zero was either equal to or close to the P-39 in speed at the altitudes of the various low level encounters.

From September to November 1942 pilots of the 57th Fighter Squadron flew P-39s and P-38s from an airfield built on land bulldozed into Kuluk Bay on the barren island of Adak in Alaska's Aleutian Islands.
They attacked the Japanese forces that had invaded Attu and Kiska islands in the Aleutians in June 1942.
The factor that claimed the most lives was not the Japanese but the weather.
The low clouds, heavy mist and fog, driving rain, snow, and high winds made flying dangerous and lives miserable.
The 57th remained in Alaska until November 1942, then returned to the United States.

While Lt.
Bill Fiedler was the only American pilot to become an ace in a P-39, many later U.S. aces scored one or two of their victories in the type.
The Airacobra's low-altitude performance was good and its firepower was impressive; regardless, it soon became a joke in the Pacific Theatre that a P-400 was a P-40 with a Zero on its tail.
The United States requisitioned 200 of the aircraft being manufactured for the UK, adopting them as P-400s (named for the advertised top speed of 400 mph (644 km/h)). After the attack on Pearl Harbor, the P-400 was deployed to training units, but some saw combat in the Southwest Pacific including with the Cactus Air Force in the Battle of Guadalcanal. Though outclassed by Japanese fighter aircraft, it performed well in strafing and bombing runs, often proving deadly in ground attacks on Japanese forces trying to retake Henderson Field. Guns salvaged from P-39s were sometimes fitted to Navy PT boats to increase firepower. Pacific pilots often complained about problems of performance and unreliable armament, but by the end of 1942, the P-39 units of the Fifth Air Force had claimed about 80 Japanese aircraft, with a similar number of P-39s lost. Fifth and Thirteenth Air Force P-39s did not score more aerial victories in the Solomons due to the aircraft's limited range and poor high altitude performance.
Airacobras first fought Japanese Zeros on 30 April 1942 in a low level action near Lae, New Guinea. From May to August 1942 combat between Airacobras and Zeros took place on a regular basis over New Guinea.  Compilation of combat reports indicates the Zero was either equal to or close to the P-39 in speed at the altitudes of the various low level encounters.
From September to November 1942 pilots of the 57th Fighter Squadron flew P-39s and P-38s from an airfield built on land bulldozed into Kuluk Bay on the barren island of Adak in Alaska's Aleutian Islands. They attacked the Japanese forces that had invaded Attu and Kiska islands in the Aleutians in June 1942. The factor that claimed the most lives was not the Japanese but the weather. The low clouds, heavy mist and fog, driving rain, snow, and high winds made flying dangerous and lives miserable. The 57th remained in Alaska until November 1942, then returned to the United States.
While Lt. Bill Fiedler was the only American pilot to become an ace in a P-39, many later U.S. aces scored one or two of their victories in the type. The Airacobra's low-altitude performance was good and its firepower was impressive; regardless, it soon became a joke in the Pacific Theatre that a P-400 was a P-40 with a Zero on its tail.
Bell P-39 Airacobra