Hughes Model D-2 (XP-73 / XA-37) Heavy Fighter / Fighter-Bomber / Fast Reconnaissance Prototype
Known under many designations, the Hughes Model D-2 suffered through its ever-changing developmental life - seeing only one prototype partially completed.
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The Hughes Model D-2 was born around 1937 to compete for the US Army Air Corp's (USAAC) contract for a twin-boom "fast fighter". Government authorities deemed the Hughes submission too slow for the role, lacking the appropriate rate-of-climb for their requirement, and subsequently handed the development contract to competitor Lockheed and their "XP-38" prototype. The XP-38 would eventually evolve to become the war-winning P-38 "Lightning" pursuit fighter, the famous single-seat, twin-engine, twin-boom "Fork-Tailed Devil as it was known to the Germans". The result of the selection, no doubt, added to Hughes' already growing suspicions that the US military would never accept a Hughes-designed aircraft for military service simply because of Hughes' involvement. Nevertheless, work on the D-2 continued as a private venture with hopes of selling the US military on a "can't miss" product in the end.
Development continued into 1938 for Hughes also saw the D-2 as a chance to set new aviation records and further establish his legacy as an aviation pioneer. At this point, the D-2 was already fleshed out as a twin-engine, twin-boom design with a centralized nacelle containing the cockpit, avionics and fuel stores. The rear of the booms held the vertical tail fins and a connecting horizontal plane - the design similar to the competing XP-38 by Lockheed with Hughes believing the company had stolen his configuration. However, the Hughes design was intended for a full crew of five (as opposed to two) and was given a "tail dragger" undercarriage design (as opposed to tricycle) - at least on paper. Additionally, the aircraft was to be constructed of Duramold, the special plywood process obtained under license by Hughes - the thought being that use of wood, in a mass production sense for the military, would not require quantities of valuable aluminum - a common material to aircraft of the period. This would prove logistically sound under the pressures of a wartime economy.
The Duramold process involved birch, high pressures and temperatures which would allow for the material to be molded into the various shapes required. The end result was a rigid structure that was lightweight. Conversely, the process required a certain degree of effort to produce which could potentially slow production on a large scale. To go along with the airframe approach, Hughes centered on a pair of high-powered engines in the Wright XR-2160 "Tornado" to which the USAAC allowed Hughes access. The engines would drive four-bladed propeller assemblies. In 1939, Hughes convinced the USAAC's Material Division of a new pursuit fighter to which a contract was signed on May 22nd, 1940. Earlier in the year, Hughes Aircraft established a new facility at Culver City, California in early 1940 to further his D-2 goal.
The original design involved a top speed of 300 miles per hour with a bomb load of up to 4,000lbs and considered for a bombing role - hence its multi-person crew. A glazed-over nose cone would assist in the level bombing role and an internal bomb bay would be installed aft of the cockpit, under the main spar of the wing assemblies. However, as would be the case throughout the D-2's life, the design wasaltered in March of 1941 to become a two-seat, long-range fighter now fitting turbosupercharged engines and featuring an operational range of 2,600 miles, a top speed of 450 miles per hour and an armament of 6 x 0.50 caliber heavy machine guns. In May of that same year, the design was changed, yet again, by the USAAC to fulfill the bomber escort role which required endurance, appropriate armament and agility. It was this sort of rocky developmental life that allowed the Model D-2 to accrue a variety of official and unofficial designations throughout its existence. The official (and original) Hughes company designation was D-2 and the in-house designation for its militarized form was to be Model D-3. The D-2 then came known under the designation of D-2A and afforded various experimental modifiers in the XD-2 and DX-2 for a time. Also at some point, a three-seat light bomber version was entertained as the Hughes Model D-5.