XB-55
Turboprop engines were first tested on modified B-47s. These aircraft were used to test the effectiveness of turboprops for bombers. This project was unsuccessful as well, but provided invaluable data on high-speed performance turboprop engines.
The XB-55 program developed out of the Boeing B-47 program, with a goal of increasing the cruising and top speeds and the operating altitude of the bomber. Despite the estimated potential of the B-47, the Bombardment Branch almost immediately began considering a replacement for it, stating, “the B-47…will be an exceptionally useful weapon until such time as a replacement can be made in this field.”
Only two months before the XB-47’s first flight, the Bombardment Branch started the XB-55 program as a replacement for the B-47 medium bomber, as well as the B-29s and B-50s still in service. Design requirements were submitted to industry in October 1947 for an airplane with a 2,000-mile radius, a 10,000-pound bomb load, and a gross weight less than 200,000 pounds.
In October of 1947 the program was officially announced with plans to develop a medium bomber. Essentially the XB-55 was larger than the B-47, using turboprop engines rather than jet engines. The new engine design, using four Allison T40-A2 turbine engines was estimated to give the XB-55 a top speed of 490 mph, but the reality was that the B-47 still had far better performance ratings.
On 01 July 1948 Boeing received the contract to develop the XB-55. The airplane submitted by Boeing was based around the Allison T40 turboprop engine. 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. The XB-55 did not withstand budget crunches of the late 1940s and was cancelled in January 1949). By this time, production of the B-47B, 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.
