Boeing XB-55

The Boeing XB-55 (Boeing Model 474) was a proposed United States replacement for the Boeing B-47 Stratojet. The concept was contained in a Request for Proposal issued by the United States Air Force in October 1947, just one month after the first flight of the B-47 prototype. Several US manufacturers responded to the RFP, and Boeing was selected from among this group, given a contract on July 1, 1948 to conduct further engineering studies. Boeing's initial approach was to mount four turboprop engines on an airframe similar to its B-47: the wing would have less sweepback; the Allison T40-A-2 engines would drive three-blade contra-rotating propellers (i.e. six blades total per engine); the engines to be mounted in nacelles hung from the wings, two per side; the landing gear to be similar to the B-47's tandem gear with outriggers retracting into the outboard engine nacelles. The XB-55 had a projected top speed of 490 mph (790 km/h) and a cruising speed of 435 mph (700 km/h), with a maximum weight of 153,000 pounds, wingspan of 135 feet, and length of 118.9 feet. [ [http://www.home.att.net/~jbaugher2/b55.html XB-55 fact sheet] ] . There was a major disagreement between the engine manufacturer and the propeller manufacturer over whether the Allison T40-A-2 drive shaft was strong enough to take the forces caused at high RPM by the propellers. Allison was predicting that it would be at least 4 years before a successful power plant would be delivered. In October 1948 a conference in Dayton, Ohio was addressing the problems of the XB-55 when it was proposed over lunch that the XB-52 (Boeing Model 464), which until that point had been planned with turboprop engines, could be equipped with the forthcoming Pratt & Whitney J57. Within a week it was clear that not only would the XB-52 outperform the XB-55, it could be flying at least a year before the XB-55 could be expected to have reliable engines. Also bearing on the decision to abandon the XB-55 program were government funding restraints and the growing realization that the B-47 was becoming more successful than first projected. On 29 January 1949 the AMC were directed to cancel the Boeing XB-55 contract.

Under a revised contract, the Boeing Project 474 was converted into the Boeing Project 479, which included study of using six J40 turbojet engines (in place of the turboprops) on a similar wing planform, but with a thicker root section. Work on detailed engineering and mockup construction was canceled, although Boeing were contracted to continue conceptual studies and wind tunnel investigations. These studies proved valuable in development of the B-52 Stratofortress, which first flew on April 15, 1952.

The XB-55 project did not result in construction of a prototype. All specifications below are based on engineering studies.

pecifications

[ [http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=2666 National Museum fact sheet on XB-55 program] ]
*Span: 135 ft 0 in
*Length: 118 ft 11 in
*Height: 33 ft 8 in
*Weight: 153,000 to 168,000 lb (estimated max. takeoff weight)
*Armament: twelve 20 mm cannons and 24,000 lb of bombs
*Engines: Four Allison T40-A-2 of 5,643 hp each
*Crew: 4 to 10
*Maximum speed: 490 mph
*Cruising speed: 450 mph
*Range: 5,000 statute miles
*Service ceiling: 42,000 ft

ee also

aircontent

lists=
* List of bomber aircraft
* List of military aircraft of the United States

related=
* B-47 Stratojet

References

Notes

Bibliography

* Jones, Lloyd S. "U.S. Bombers, B-1 1928 to B-1 1980s". Fallbrook, CA: Aero Publishers, 1962, second edition 1974. ISBN 0-8168-9126-5.