Specifications called for at least 1,000 lb (450 kg) of heavy armament including a cannon, a liquid-cooled Allison engine with a General Electric turbo-supercharger, tricycle landing gear, a level airspeed of at least 360 mph (580 km/h) at altitude, and a climb to 20,000 ft (6,100 m) within six minutes.

[11] The main purpose of this configuration was to free up space for a 37 mm Browning Arms Company T9 cannon, later produced by Oldsmobile, firing through the center of the propeller hub for optimum accuracy and stability.

The XP-39 project was handed over to others, and in June 1939 the prototype was ordered by General Henry H. Arnold to be evaluated in NACA wind tunnels to find ways of increasing its speed, by reducing parasitic drag.

The Airacobra was one of the first production fighters to be conceived as a "weapons system"; in this case the aircraft (known originally as the Bell Model 4) was designed to provide a platform for the 37 mm T9 cannon.

[29] This weapon, which was designed in 1934 by the American Armament Corporation, a division of Oldsmobile, fired a 1.3 lb (0.59 kg) projectile capable of piercing .8 in (20 mm) of armor at 500 yd (460 m) with armor-piercing rounds.

The tractor propeller was driven with a 10-foot-long (3.0 m) drive shaft made in two sections, incorporating a self-aligning bearing to accommodate fuselage deflection during violent maneuvers.

[29] The glycol-cooled radiator was fitted in the wing center section, immediately beneath the engine; this was flanked on either side by a single drum-shaped oil cooler.

[34] Above the supercharger's peak altitude of about 12,000 ft (3,700 m), performance dropped off rapidly, limiting usefulness in traditional fighter missions in Europe as well as in the Pacific, where it was not uncommon for Japanese bombers to attack from above the P-39's ceiling (which in the tropical heat was lower than in cooler climates).

Weight distribution could result in it entering a dangerous flat spin, a characteristic Soviet test pilots demonstrated to the skeptical manufacturer, which had been unable to reproduce the effect.

However, after the first Airacobras arrived at 601 Squadron RAF in September 1941, they were found to have an inadequate rate of climb and performance at altitude for Western European conditions.

Another 200 examples intended for the RAF were taken up by the USAAF after the attack on Pearl Harbor as the P-400, and were sent to the Fifth Air Force in Australia, for service in the South West Pacific Theatre.

In 1940, the British Direct Purchase Commission in the U.S. was looking for combat aircraft; they ordered 675 of the export version Bell Model 14 as the Caribou on the strength of the company's representations on 13 April 1940.

The squadron continued to train with the Airacobra during the winter, but a combination of poor serviceability and deep distrust of this unfamiliar fighter resulted in the RAF rejecting the type after one combat mission.

The Airacobras already in the UK, along with the remainder of the first batch being built in the US, were sent to the Soviet Air force, the sole exception being AH574, which was passed to the Royal Navy and used for experimental work, including the first carrier landing by a tricycle undercarriage aircraft, on 4 April 1945 on HMS Pretoria Castle,[48] until it was scrapped on the recommendation of a visiting Bell test pilot in March 1946.

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.

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.

The low clouds, heavy mist and fog, driving rain, snow, and high winds made flying dangerous and lives miserable.

The 99th carried out their duties including supporting Operation Shingle over Anzio as well as missions over the Gulf of Naples in the Airacobra but achieved few aerial victories.

The comparatively low-speed, low-altitude nature of most air combat on the Eastern Front suited the P-39's strengths: sturdy construction, reliable radio gear, and good firepower.

Soviet P-39s had no trouble dispatching Junkers Ju 87 Stukas or German twin-engine bombers and matched, and in some areas surpassed, early and mid-war Messerschmitt Bf 109s.

[63] Airacobras served with the Soviet Air Forces as late as 1949, when two regiments were operating as part of the 16th Guards Fighter Aviation Division in the Belomorsky Military District.

[65] In the early months of the Pacific War, the RAAF was able to obtain only enough Curtiss Kittyhawks to equip three squadrons, destined for front-line duties in New Guinea.

[66] and – in the face of increasing Japanese air raids on towns in northern Australia – was forced to rely on the P-40, P-39, and P-400 units of 5 AF for the defence of areas such as Darwin.

In February–March 1945, 10° and 9° Gruppi moved North of Galatina, in Canne airbase, near Campobasso, while Allied allowed Italian pilots to use the airstrip of Lissa island, in the Adriatic sea, as an intermediate scale during the long sorties on the Balkans.

The 4° Stormo pilots flew many effective ground attack missions on northern Yugoslavia, losing only one more P-39, for engine failure in Sarajevo area, on 2 April 1945.

[70] By the end of the war, 89 P-39s were still at the Canne airport and 13 at the Scuola Addestramento Bombardamento e Caccia ("Training School for Bombers and Fighters") at Frosinone airfield.

In Galatina fighter training unit (Scuola Caccia), war veteran Tenente colonnello Francis Leoncini was killed during a flying accident, on 10 May 1950.

The "Cobra II" flown by test pilot "Tex" Johnston, beat racing-modified P-51s, as well as other P-39 racers (which were the favorites), to win the 1946 Thompson Trophy race.

Cobra II did not race again and was destroyed on 10 August 1968 during a test flight prior to an attempt at the world piston-engine air speed record, when owner-pilot Mike Carroll lost control and crashed.