In October of 1947, a request was issued to the aviation industry for a new medium bomber that would be the successor to the B-47, the prototype of which had made its first test flight only a month earlier. The program was assigned the bomber designation of XB-55.
On July 1, 1948, Boeing was named the winner of the contest and was granted a contract. The Boeing entry was assigned the company designation of Model 474. The Model 474 was essentially a turboprop adaptation of the jet-powered XB-47. It was to be powered by four 5643 hp Allison T40-A-2 turboprops housed in individual pods slung below a slightly-swept high-mounted wing. The turboprops drove a set of three-bladed contrarotating propellers. The landing gear configuration was similar to that of the XB-47--a tandem pair of wheels which retracted into the fuselage and supported by outrigger wheels which retracted into the outer engine nacelles.
Gross weight was estimated at 153,000 pounds. Defensive armament was to consist of twelve 20-mm cannon housed in three separate turrets which were mounted in the rear of the aircraft and directed remotely by an aft-mounted gunner's compartment. The wingspan was to be 135 feet, and the length was to be 118 feet 11 inches. However, the maximum speed was projected to be only 490 mph, much slower than the standard B-47.
The Model 474 later metamorphosed into the Model 479, powered by six Westinghouse J40 turbojet engines and featuring a thickened wing root section.
In January of 1949, the XB-55 project was cancelled. One reason for the cancellation was a lack of money caused by the Fiscal Year 1949 budgetary crisis, which caused the termination of several military projects and the delay of others. In addition, there no longer seemed to be any immediate need to develop a new medium bomber, in view of the currently-projected growth in the B-47. The XB-55 project promised to take much longer than originally expected, and the Air Force thought that its design should have been based on more-advanced aerodynamic principles as well as on improved propulsion systems. The mockup and detailed engineering work then taking place on the XB-55 were halted, but the Air Force allowed the study reports and wind tunnel testing to continue. These were to prove useful in the development of the XB-52, which was also in development at the same time at Boeing.
The turboprop engine was eventually tested on a USAF bomber when two B-47s were modified for Wright YT49 turboprops. The aircraft was designated XB-47D and was used to test the feasibility of using turboprop engines for bombers. Although the project was unsuccessful, it did provide some valuable test date on the high-speed performance of turboprop engines.