Allison was a key supplier of aircraft engines to the US Army and took to development of a new series product under the designation of V-3420, developed from the mating of two V-1710 series engines. In theory it was a vastly powerful engine but it remained wholly untested in the real world war time market. Nevertheless, a new product warranted equally new sales for the division and General Motors was not going to let its new engine development lay by the wayside - not with a world war going on.
By 1942, development of the war was not without its growing pains for the US military. Though it was already putting together a successful stead of legendary "pursuit" fighters in the North American P-51 Mustang, the Republic P-47 Thunderbolt and the Lockheed P-38 Lightning, these aircraft would generally not reach their true pinnacles for a few more years to come. As such, the need was apparent for better performing, high-flying and lethal interceptors to help stave off any aerial advantage that lay in the German Luftwaffe to this point. As such, a fighter needed to be developed that could provide a lethal punch against both fighter or bomber and could respond to incoming threats through excellent climbing abilities and top speed.
General Motors saw this as the perfect opportunity to showcase the new Allison powerplant and moved to convert their Fisher Body Plant in Cleveland, Ohio for production of not only the Allison engine but also of a new interceptor to fill the US Army need. Accordingly, they penciled out a design specifically fitting their powerplant and submitted it for review. Amazingly, the US Army took notice and accepted the General Motors idea - moreso in terms that the new fighter could be rapidly produced while fulfilling the performance requests as needed by the Army. The contract called for two complete prototypes and the designation of XP-75 was assigned.
To keep design and development of the new system short, the idea presented was to build an aircraft out of proven portions of existing airframes with the entire concoction fitted around the Allison V-3420. Outer wing surfaces were pulled from a P-51 Mustang while the wings themselves were originally slated to resemble the inverted "gull wing" implements as found on the Vought F4U Corsair fighter. The inverted gull wings were dropped from the design and, instead, the wings of a Curtiss P-40 Warhawk was used in its place though the undercarriage of the F4U was still selected for use. The empennage was to be made up from the system of a Douglas Dauntless dive bomber.
The final result became one of the most unique - if not dysfunctional-looking - aircraft designs to emerge from World War 2. The fuselage became a long slender affair with the cockpit situated quite well-ahead of the low-mounted monoplane wings. The nose protruded out some and was capped with a contra-rotating set of three-bladed propellers (giving the appearance of six blades in the nose). The design called for the Allison powerplant to be fitted at the midway point of the fuselage with a drive shaft running under the cockpit floor and attached to each propeller system. As a liquid-cooled engine, the V-3420 had no need for air cooling scoops and could be fitted as such. The radiator was fed via two scoops mounted under the rearward portion of the fuselage. The empennage maintained curved surface features and was topped with a smallish sort of vertical tail fin and accompanying horizontal planes. The undercarriage consisted of two main landing gears - one mounted under each wing - and a diminutive tail wheel. The canopy was heavily framed at first and offered up adequate views all-around the airframe. In whole, the XP-75 was constructed over in all-metal stressed skin.
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