Douglas XB-31 (Raidmaster) (Model 332) Super-Heavy Bomber Aircraft Design Proposal
The Douglas XB-31 super heavy bomber was proposed against the Boeing XB-29, the Lockheed XB-30, and the Consolidated XB-32 - it was not furthered beyond a design study.
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The Douglas XB-31 "Raidmaster" became one of four submissions passed along to the United States Army Air Forces (USAAF) during World War 2 (1939-1945) attempting to fulfill a requirement for a new long-range, high-altitude "super heavy bomber". The requirement was eventually filled by the Boeing B-29 "Superfortress" with Consolidated's XB-32 selected as an insurance measure (this becoming the B-32 "Dominator"). Before the decision was made, the XB-31 competed against the Boeing submission as did an entry from Lockheed (the XB-30, detailed elsewhere on this site) - both were removed from contention in time.
Origins of the XB-31 lay in the late 1930s when American authorities realized advancements being made in military aircraft in Europe (particularly Germany) were beyond anything that was had in the current inventory. A committee was arranged by the Army for direction and the consensus was to pursue a new very-heavy, long range bomb delivery platform. The outbreak of war in Europe during September 1939 only served to put an emphasis on getting the large aircraft into the sky in short order.
The requirement called for exceptional range and excellent operating altitudes, the latter to help keep the system as far away from enemy interceptors and ground-based fire as possible. Speed was also essential as was a competent bomb load to make the product worth its investment. To this point, the standard heavy bombers in the American stable were the Boeing B-17 "Flying Fortress" (introduced in 1938) and the Consolidated B-24 "Liberator" (introduced in 1941) - both classics in their own right but eventually outmoded by the advancing nature of the war.
As such, work proceeded on finding their successor - Boeing held a head start on their XB-29 product and eventually won the contract. In 1940, Douglas readied theirs through the "Model 332" initiative to which the U.S. Army designation became "XB-31". On paper, the Douglas submission already surpassed the other three entries in terms of size and operating weight - a mammoth design to be sure.
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.