The Boeing Y1B-20 (Boeing 316) was designed as an improvement on the
Boeing XB-15.[a] It was slightly larger than its predecessor, and was intended to use much more powerful engines. It was presented to the Army in early
1938, and two orders were placed soon after. The order was reversed before construction began.
^Y1- indicates a funding source outside normal fiscal year procurement.
^Kohn, Leo. "Boeing XB-15 Super Flying Fortress: U.S. Heavy Bomber". In Ray Merriam (ed.). U. S. Warplanes of World War II. World War II Journal. Vol. 69. Merriam Press. pp. 43–44.
ISBN1-57638-167-6.