The USAF’s Bombardment Branch started the XB-55 program to replace the B-47 medium bomber, as well as the B-29s and B-50s.

The XB-55 program developed out of the Boeing B-47 program, with a goal of increasing the cruising and the operating altitude of the aircraft. Despite the high potential of B-47, the Bombardment Branch began considering a replacement right after it was developed. 

In October 1947, just two months before the XB-47’s first flight took the sky, the Bombardment Branch Design submitted configurations for an airplane with a 2,000-mile radius, a 10,000-pound bomb load, and a gross weight less than 200,000 pounds; to the airline industry. The Bombardment Branch Design wanted an aircraft that has turboprop engines. On July 1, 1948, Boeing was named as the manufacturer of such an aircraft. The Boeing entry was assigned the company designation of Model 474 which was a turboprop adaptation of the jet-powered XB-47. It was designed to be powered by four 5643 hp Allison T40-A-2 turboprops, incorporated in individual pods slung below a slightly-swept high-mounted wing. The new engine design, using four Allison T40-A2 turbine engines theoretically gave the XB-55 a top speed of 490 mph, but in reality - the B-47 still had far better performance ratings. 

On 01 July 1948 Boeing received the contract to develop the XB-55. The original requirement for all-around defensive armament was relaxed to allow for nose and tail armament only, a factor that reduced the weight of the airplane to allow it to meet the range requirement. Over the next few years, the XB-55 program evolved into a paper study on a turbojet configuration, and investigations were even conducted on a delta-wing configuration. But due to the budget constrain in the late 1940’s; the Air Force decided to stay with the B-47B as its medium bomber and cancelled the production of the B-47B.