The Douglas Model 423 was a bomber aircraft design developed by American aircraft manufacturer Douglas to compete with the Convair B-36 design for a major U.S. Army Air Force contract for an intercontinental bomber in 1941.
Although identified as the Douglas XB-31 in some publications, the project documents indicate that it was designed much later than the R40-B competition.
In April 1941, the possibility of Great Britain falling to Nazi Germany seemed very real, and so the United States Army Air Corps unveiled a competition for a long-range bomber with intercontinental range (10,000 miles), making it capable of conducting air-strikes on Nazi-occupied Europe from US bases.
(Note: The primary source labels this project as the XB-31, which was much smaller, earlier project, competing with the B-29 and B-32) Data from McDonnell Douglas aircraft since 1920 : Volume I (erroneously labelled as XB-31)[2]General characteristics Performance Armament
Aircraft of comparable role, configuration, and era