Staff Writer (Updated: 4/18/2016):
The P-39 Airacobra was already in production for the USAAF . This unique little aircraft sported a rear-mounted engine turning a three-bladed propeller system via a shaft running under the cockpit floor. With the engine mounted in the rear of the fuselage, the nose assembly was free to fit armament - this coming in the form of a 37mm Oldsmobile M4 cannon firing unobstructed through the propeller hub. Additionally 2 x 12.7mm heavy machine guns were fitted into the engine cowl and synchronized for firing through the spinning propeller blades. This armament was supplemented by the addition of four .30 caliber machine guns - two to a wing. Power was supplied by an Allison-brand V-1710-85 liquid-cooled V-12 engine supplying up to 1,200 horsepower.
Though initially unveiled as a dedicated interceptor, the uniqueness of the aircraft proved much more expensive than the USAAF was interested in and, as such, it provided "suggestions" in an attempt to cut down on costs. One of the biggest changes was in dropping the turbocharger - effectively the backbone of the P-39's performance as an interceptor. This was replaced by a mechanically-based supercharger which didn't provide much in the way of high-altitude performance and the P-39 was doomed as a fighter. As it stood, the P-39 went on to more action in the low-altitude attack aircraft, with most of its success coming from the hands of Soviet pilots utilizing the type through Lend-Lease deliveries. Despite its inherent deficiencies, the P-39 became Bell's most successful production endeavor.
While the P-39 design essentially foundered for what it was intended to be, Bell sought solutions to the design while USAAF interest was still in their favor. The same basic fuselage design and engine (with two superchargers) arrangement was taken and fitted into an enlarged airframe - essentially based on the XP-39E with a laminar flow wing planform, improved supercharger and new Continental I-1430 engine. Three prototypes were ordered with two fitting the base Allison V-1710-47 series engine and one fitting a version of the Rolls-Royce Merlin engine built by Packard as the V-1650 series (Merlins were used in the excellent Supermarine Spitfires and North American P-51 Mustangs). The Packard-engined XP-63 was never to be for these Merlins were earmarked for Mustangs, leaving this prototype to eventually be powered by an Allison V-1710-93 engine. These engines powered a new large four-bladed propeller system. Armament remained faithful to the P-39Q model design with its 37mm nose cannons, 12.7mm heavy machine guns in the engine cowl and an additional 2 x 12.7mm heavy machine guns slung under the wings in pods. The developmental designation was set as XP-63.
Externally, the P-63 remained faithful to the P-39 Airacobra design before it with the exception of its larger overall dimensions. The pilot sat relatively forward along the contoured fuselage in a glazed cockpit featuring the identifiable automobile-style doors of the P39 design. The cockpit was situated just forward of the wing leading edge. Wings were rounded monoplanes with noticeable dihedral. The front of the fuselage was dominated by the large four-bladed propeller system. The propeller was controlled via a shaft running under the cockpit floor and into the rear-mounted Allison engine, this being situated about midway through the fuselage. The empennage was of a standard configuration with a single vertical tail fin and a conventional horizontal plane. The undercarriage was a more-modern tricycle arrangement featuring two main single-wheeled landing gears and a single-wheeled nose gear. The nose gear retracted rearwards under the cockpit floor whilst the main gears retracted inwards towards the fuselage centerline, each system fitted under each wing. When at rest, the P-63 took on a pronounced "front-up" look thanks to its tall nose gear. Overall, she was a fine-looking - albeit unique - aircraft with much of her P-39 pedigree still in place. As mentioned, armament was carried over from the P-39Q model series with some slight alterations in placement and ammunition supply in the P-63.