Boeing XB-55 Propeller-driven version of the B-47 Stratojet (Includes as bonus, YF17 Cobra, F5D Skylancer, XV3 Tiltrotor)

In October 1947, the USAF announced the medium swept-wing bomber program with plans essentially using the new Allison T40-A2 turbprop engines design. It was estimated to give the new bomber a top speed of 490 mph. Boeing submitted the Model-474 design that was started as a replacement for the XB-47 Stratojet. In October 1948, Boeing received the contract to develop the new bomber which designated XB-55. Over the next few years, the XB-55 program evolved into a paper study on a turbojet configuration. It did not withstand budget crunches of the late 1940s. By that time, production of the B-47A with accommodations to carry a nuclear bomb, and begun, and the Air Force instead decided to stay with the B-47B as its medium bomber. The XB-55 was cancelled in 1949. The turboprop engine was eventually tested on a USAF bomber when two B-47s were modified for Wright YT49 turboprops. The aircraft, designated XB-47D, were used to test the feasibility of using turboprops for bombers. This project was unsuccessful also, but did provide some valuable test data on the high speed performance of turboprop engines