Not convinced of the reliability of, and promised performance from, early turbojet engines, the United States Air Force (USAF) looked beyond all-jet-powered forms in-development after the close of World War 2 (1939-1945). The Boeing B-47 "Stratojet" swept-wing strategic jet bomber was selected by the USAF to fulfill an earlier specification and was a product of the period - it eventually went on to find success as over 2,000 of the type were built and flown until the late-1970s.
However, even before the B-47 prototype went airborne in 1947, USAF authorities penciled out a new request in October of that year for an insurance-minded strategic bombing platform relying on more conventional propulsion power. The usual participants responded and this crop included Boeing whose submission was selected for further development under the designation of "XB-55" (Company Model 474).
Boeing engineers elected for a form that was not unlike their B-47 proposal save for the use of Allison T40-A-2 turboprop engines over turbojets. These systems would be underslung at the wings in the usual way (the wing mainplanes being of reduced sweepback when compared to the B-47) and drive three-bladed contra-rotating propeller units. Four engines would be used to power the large bomber and output up to 5,600 horsepower. Estimated performance included a maximum speed of 490 miles per hour (cruising at about 450 mph), a range out to 5,000 miles and a ceiling of 42,000 feet. In comparison, the B-47 (B-47E) managed a maximum speed of 607mph, cruised at 557mph, flew out to 4,650 miles and reached up to 33,100 feet of altitude.
The XB-55 would be crewed by ten personnel and the airframe was to hold an overall length of 119 feet, a wingspan of 135 feet and a height of 33.7 feet. Maximum Take-Off Weight (MTOW) was listed at 168,000lb. As in the B-47, the XB-55 was to feature a "bicycle" undercarriage arrangement in which the main legs were held inline under the mass of the body and outrigger legs were used to support the wings. Standard, defensive armament was to include 10 x 20mm cannons while the bomb load reached 24,000lb of stores. The B-47 was defensed by 2 x 20mm cannons and carried around 25,000lb of stores.
In many ways the XB-55 was an insurance policy against the B-47 - it was far more conventional in design and held qualities for a bomber influenced by the recent World War but the USAF could at least understand what it was getting without pinning all their hope on the success (or failure) of unreliable, underpowered and thirsty turbojet engines of the time.
Eventually there proved delays with the expected Allison engines which prompted a look into an all-turbojet-powered form by way of the "XB-52" initiative - this design promised considerably better performance figures from the impending arrival of the Pratt & Whitney J57 turbojet engine. Couple this with waning interest on the part of the USAF over the XB-55 (the B-47 was seeing success in service by this point) and the XB-55 project contract was terminated in 1949. It, too, dawned on the USAF that it required a fleet of fast bombers and turboprop-powered forms were not going to be sufficient.
The XB0-55 endeavor lived a short time longer as "Model 479" which studied the use of 6 x Westinghouse J40 turbojet engines along a revised wing element and this data then served the upcoming Boeing B-52 "Stratofortress" strategic heavy bomber program. Leftover funding for the XB-55 went into pursuit of a supersonic strategic bomber under the Boeing "XB-59" designation (Model 701).
As no functional prototype of the XB-55 was ever completed, specifications reported in this article and on this page are purely estimates based on engineering data.
(OPERATORS list includes past, present, and future operators when applicable)
✓Ground Attack (Bombing, Strafing)
Ability to conduct aerial bombing of ground targets by way of (but not limited to) guns, bombs, missiles, rockets, and the like.
✓X-Plane (Developmental, Prototype, Technology Demonstrator)
Aircraft developed for the role of prototyping, technology demonstration, or research / data collection.
118.9 ft (36.25 m)
135.0 ft (41.15 m)
33.6 ft (10.25 m)
99,208 lb (45,000 kg)
167,992 lb (76,200 kg)
+68,784 lb (+31,200 kg)
(Showcased structural values pertain to the base Boeing XB-55 production variant)
4 x Allison T40-A-2 turboprop engines developing 5,600 horsepower each and driving three-bladed propellers in contra-rotating fashion.
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