Douglas engineers elected for a conventional arrangement which sat the flight deck over the nose, the monoplane wings slightly ahead of midships, and a single rudder affixed to the tail. The wing mainplanes were shoulder mounted and each given a pair of underslung engine nacelles. The fuselage exhibited a very streamlined appearance with a very pointed nose section and tapered tail section promoting excellent aerodynamic qualities. The two bomb bays resided in the belly in the usual way. The tail unit incorporated a large-area fin. A tricycle undercarriage was featured and fully retractable into the airframe. Due to the high-altitude operation expected of this massive bomber, crew stations were pressurized. The standard operating crew would number eight. There existed a dorsal and ventral turret, each remotely-controlled by an operator, and armed through 2 x 0.50 caliber heavy machine guns. The tail unit carried a trainable set of 37mm cannons to protect the aircraft's rear from interceptors. The internal bomb load measured 25,000lb. Dimensions were a wingspan of 207 feet, a length of 117.2 feet and a height of 42,6 feet.
Power for the design was to originally come from 4 x Wright R-3350-13 "Duplex-Cyclone" radial piston engines of 2,200 horsepower each. This was later changed (through a redesign) to 4 x Pratt & Whitney R-4360 "Wasp Major" radial piston engines outputting 3,000 horsepower each. Estimated performance specifications for the bomber included a maximum speed of 360 miles per hour, a range out to 3,000 miles, and a service ceiling of 35,000 feet.
Despite the promising nature of the Douglas entry, Boeing's progress with its XB-29 (and Consolidated with their XB-32) was such that the Douglas and Lockheed submissions were dropped from contention. As such, the XB-31 only ever existed as a design study and nothing more.
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