Boeing Y1B-20
The Boeing Y1B-20 was designed as an improvement on the Boeing XB-15. 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.
Despite their cancellation, the XB-15 and Y1B-20 laid the groundwork for the Boeing B-29 Superfortress.