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The Douglas XB-31 was the designation given to a series of Douglas designs produced as part of the same design contest that produced the Boeing B-29 Superfortress, none of which were ever built.
Some published sources suggest that the Douglas Model 423 was submitted in responce to this design contest. The identification of the XB-31 with the Model 423 is unlikely to be correct. Alan Griffith, who is currently working with the Douglas archives, has provided convincing evidence that the Douglas Model 332 (and in particular the Model 332F) was in fact the aircraft submitted as the XB-31. This was a four engined bomber, powered by the R-3360 Duplex Cyclone engine. Drawings show a high winged aircraft, with notable dihedral on the tapered wings, a circular cross-section fuselage, a standard bomber canopy, glazed nose and a massive vertical tail. This dates to May 1940, the same month in which Boeing submitted their Model 345, the basis for the B-29. Documents also survive that compare the Douglas 332F to the Lockheed entry in the contest. The Model 332F had a wingspan of 140ft 8.5" and a length of 88ft 8.5", making it similar in size to the B-29. The Model 332F didn't progress beyond the design stage.
The later Model 423 would have been similar in size to the massive Douglas XB-19, with a slightly reduced 207ft wing span but a higher maximum loaded weight of 198,000lb. It would have been powered by four 3,000hp Pratt & Whitney R-4360 Wasp Major engines, giving it much more power than the B-19. It would have had a fuselage with a circular cross section, tapered wings (with tapered leading and trailing edges) and a massive single tail fin. Some sources suggest that the Model 423 was based on the DC-4 Skymaster. If so the connections were rather limited, perhaps only including the circular cross section of the fuselage and general shape of the wings. The Model 423 had a crew of 8. The pilot and co-pilot sat under separate bubble canopies carried side by side near the nose. The aircraft was armed with two 37mm cannon in the tail and four 0.50in guns carried in pairs in remote controlled dorsal and ventral turrets. It would have carried 25,000lb of bombs in two internal bomb bays. Work on the Model 423 came to an end late in 1941, although some of its design features appeared on later aircraft - the double bubble canopies were used on the C-74 transport and the tail on the Douglas A-26 Invader attack bomber (although massively reduced in scale).
The exact use of the Model 423 is unclear. It may have been developed in responce to the 1941 specifications that produced the massive Consolidated B-36, or as an internal Douglas response to the failure to win the B-29 contract, or possibly as an improved version of the Model 332. The Model 423's wingspan of 207ft was nearer to the 230ft of the B-36.