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 1.3 lb (590 g) projectile capable of piercing .8 in (2.0 cm) of armor at 500 yd (460 m) with armor-piercing rounds.
The 90 in (230 cm)-long, 200 lb (91 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 20mm 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 10-foot-long (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.

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
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 1.3 lb (590 g) projectile capable of piercing .8 in (2.0 cm) of armor at 500 yd (460 m) with armor-piercing rounds. The 90 in (230 cm)-long, 200 lb (91 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 20mm 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 10-foot-long (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.
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
Bell P-39 Airacobra