Originally designated as the XBL-2, the XB-19 was Douglas Aircraft's response to the US Army Air Corps "Project D" requirement for a heavy bomber able to carry a one-ton bomb load over a range of 5,000 miles. Substantially larger than the later Boeing Superfortress, the XB-19 was the largest American bomber until the advent of the Convair B-36. First flown in June 1941, the XB-19 had a tremendous range of over 7,000 miles, and a maximum war load of 18 tons.
XB-19 3-view, from US patent documents
The XB-19's rotating nose turret installation, resembling that of the far smaller B-18 Bolo by the same company, held a 37mm cannon; there were also a pair of retractable dorsal turrets, one with another 37mm, the other with a .50-cal machine gun.
Even before the XB-19 flew, both the USAAC and Douglas recognized that it would be a one-off technology demonstrator. The R-3350, powerful as it was, was too small for the huge, heavy airframe, giving an inadequate cruise speed of 140 mph. The R3350 was also still early in its development cycle, and suffered from overheating.
In 1944, long after the "Hemisphere Defender" had faded into obscurity, the aircraft was reengined as the XB-19A with Allison V-3420 liquid cooled engines. these powerplants boosted the top speed to 265mph, but the end was not far off for the one-off prototype, and after a stint with the All-Weather Flying Center, it took one last flight in 1946 to Davis-Monthan for storage. there it sat until being broken up for scrap in the summer of 1949.
Anigrand makes a 1/144th scale resin kit of the XB-19.
Photo: Close-up view of the XB-19's tail. [Ryan School of Aeronautics ad] Flying August 1941 p.89
Edward Churchill "What We learned from the B-19" Flying July 1942
Dan McCarthy, Fred Johnsen "Behemoth - Douglas XB-19...Data Bank for All Future Flying Giants" Airpower March 1975