General Henry H. (Hap) Arnold, the acting head of the US Army Air Corps, had become alarmed by the growing war clouds in Europe and in the Far East. He established a special committee, chaired by Brigadier General W. G. Kilner, to make recommendations for the long term needs of the Army Air Corps. No less a personage than the famous aviator Charles Lindbergh had been a member of the committee. Lindbergh had recently toured German aircraft factories and Luftwaffe bases, and had become convinced that Germany was well ahead of its potential European adversaries in aeronautical technology.
In their June 1939 report, the Kilner committee recommended that several new long-range medium and heavy bombers be developed. Hastened by a new urgency caused by the outbreak of war in Europe on September 1, on November 10, 1939, General Arnold requested authorization to contract with major aircraft companies for studies of a Very Long-Range (VLR) bomber that would be capable of carrying any future war well beyond American shores. Approval was granted on December 2, and USAAC engineering officers under Captain Donald L. Putt of the Air Material Command at Wright Field began to prepare the official specification.
In January of 1940, the Army issued requirements for a "superbomber" with a speed of 400 mph, a range of 5333 miles, and a bomb load of 2000 pounds delivered at the halfway-point at that range. The official specification was revised in April to incorporate the lessons learned in early European wartime experience, and now included more defensive armament, armor, and self-sealing tanks. This became the basis for Request for Data R-40B and Specification XC-218. On January 29, 1940, the War Department formally issued Data R-40B and circulated it to Boeing, Consolidated, Douglas, and Lockheed.
On June 27, 1940, the Army issued contracts for preliminary engineering data for the new "superbomber" to four manufacturers, which were designated in order of preference as Boeing XB-29, Lockheed XB-30, Douglas XB-31, and Consolidated XB-32.
The Douglas XB-31 (company designation of Model 332) was somewhat larger and heavier than the other three competitors. It had a high-mounted wing, and was to have been powered by four Wright R-3350 air-cooled radials driving three-bladed propellers. It had a single vertical tail. The pilot and copilot were seated in conventional side-by-side seating arrangements. The six other crew members were to be accommodated at separate stations throughout the fuselage. Defensive armament was to have consisted of twin 0.50-inch machine guns in each of four remotely controlled fuselage turrets (two on the top and two on the bottom), plus two 0.50-inch machine guns mounted in the rear of each outboard engine nacelle, plus a pair of 0.50-inch machine guns in the tail. There were no nose guns. Dimensions were to have been wingspan 140 feet 8 1/2 inches, length 88 ft 6 1/2 inches and height 28 feet 3 inches. A maximum bombload of 25,000 pounds was to have been carried in two fuselage bays.
In spite of the promise of the XB-31, the B-29 had the edge in the competition since work on the Boeing design was much further along. On May 17, 1941, the Army announced that an order would be placed for 250 B-29s. This order was confirmed in September of 1941. The Douglas XB-31 project was formally cancelled in late 1941 before anything could be built.