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The P-39 was an all-metal, low-wing, single-engine fighter, with a tricycle undercarriage and an Allison V-1710 liquid-cooled V-12 engine mounted in the central fuselage, directly behind the cockpit.

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. This weapon, which was designed in 1934 by the American Armament Corporation, a division of Oldsmobile, fired a 0.59 kg projectile capable of piercing 20 mm of armor at 460 m with armor-piercing rounds. The 2.3 m, 90 kg weapon had to be rigidly mounted and fire parallel to and close to the centerline of the new fighter. It would have been impossible to mount the weapon in the fuselage, firing through the cylinder banks of the Vee-configured engine and the propeller hub as could be done with smaller 20 mm cannon. Weight, balance and visibility considerations meant that the cockpit could not be placed farther back in the fuselage, behind the engine and cannon. The solution adopted was to mount the cannon in the forward fuselage and the engine in the center fuselage, directly behind the pilot's seat. The tractor propeller was driven with a 3.0 m drive shaft made in two sections, incorporating a self-aligning bearing to accommodate fuselage deflection during violent maneuvers. This shaft ran through a tunnel in the cockpit floor and was connected to a gearbox in the nose of the fuselage which, in turn, drove the three- or (later) four-bladed propeller by way of a short central shaft. The gearbox was provided with its own lubrication system, separate from the engine; in later versions of the Airacobra the gearbox was provided with some armor protection. 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. Air for the radiator and oil coolers was drawn in through intakes in both wing-root leading edges and was directed via four ducts to the radiator faces. The air was then exhausted through three controllable hinged flaps near the trailing edge of the center section. Air for the carburetor was drawn in through a raised oval intake immediately aft of the rear canopy.

The fuselage structure was unusual and innovative, being based on a strong central keel that incorporated the armament, cockpit, and engine. Two strong fuselage beams to port and starboard formed the basis of the structure. These angled upwards fore and aft to create mounting points for the T9 cannon and propeller reduction gearbox and for the engine and accessories respectively. A strong arched bulkhead provided the main structural attachment point for the main spar of the wing. This arch incorporated a fireproof panel and an armor plate between the engine and the cockpit. It also incorporated a turnover pylon and a pane of bullet-resistant glass behind the pilot's head. The arch also formed the basis of the cockpit housing; the pilot's seat was attached to the forward face as was the cockpit floor. Forward of the cockpit the fuselage nose was formed from large removable covers. A long nose wheel well was incorporated in the lower nose section. The engine and accessories were attached to the rear of the arch and the main structural beams; these too were covered using large removable panels. A conventional semi-monocoque rear fuselage was attached aft of the main structure.


The weapons bay of the P-39
Because the pilot was above the extension shaft, he was placed higher in the fuselage than in most contemporary fighters, which, in turn gave the pilot a good field of view. Access to the cockpit was by way of sideways opening "car doors", one on either side. Both had wind-down windows. As only the right-hand door had a handle both inside and outside this was used as the normal means of access and egress. The left-hand door could be opened only from the outside and was for emergency use, although both doors could be jettisoned. In operational use, as the roof was fixed, the cockpit design made escape difficult in an emergency.


Because of the unconventional layout, there was no space in the fuselage to place a fuel tank. Although drop tanks were implemented to extend its range, the standard fuel load was carried in the wings, with the result that the P-39 was limited to short-range tactical strikes.

A heavy structure, and around 256 lb (116 kg) of armor, were characteristic of this aircraft as well. The production P-39's heavier weight combined with the Allison engine with only a single-stage, single-speed supercharger, limited high-altitude performance, which was markedly inferior to contemporary European fighters and, as a result, the first USAAF fighter units in the European Theater were equipped with the Spitfire V. However, the P-39D's roll rate was 75°/s at 235 mph (378 km/h)– better than the A6M2, F4F, or P-38 up to 265 mph (426 km/h).[34]

Above the supercharger's peak altitude of about 3,660 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). The late production N and Q models, which made up 75% of Airacobras built, could maintain a top speed of 604 km/h up to 6,100 m.

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. It was determined the spin could only be induced if the aircraft was flown with no ammunition in the nose. The flight manual noted a need to ballast the front ammunition compartment to achieve a reasonable center of gravity. High-speed controls were light, consequently high-speed turns and pull-outs were possible. The P-39 had to be held in a dive since it tended to level out and the recommended never-exceed dive speed limit (Vne) was 764 km/h
The rear-mounted engine was less likely to be hit when attacking ground targets, but was vulnerable to attacks from above and behind. At its upper altitude limits, the Airacobra was out-performed by many enemy aircraft.