Design and development
In 1944, the U.S. War Department was aware of aviation advances in Germany and issued a requirement for a range of designs for medium bombers weighing from 80,000 pounds (36,000 kg) to more than 200,000 pounds (91,000 kg). Other designs resulting from this competition, sometimes nicknamed "The Class of '45", included the North American XB-45 and the Convair XB-46.
All of the bombers comprising the Class of '45 were transitional aircraft, which combined the power of turbojets with the aeronautical design of World War II, notably the unswept wings and tail surfaces. The XB-48 was no exception, as its round fuselage and unswept wings showed a distinct influence of Martin's B-26 Marauder medium bomber.
At the time of the XB-48's design, jet propulsion was still in its infancy. While the B-26 had enough thrust with two massive 18-cylinder radial engines, the XB-48 needed no less than six of the low-thrust turbojets available at the time. And, although it appeared superficially to have six separate engine nacelles – that is, three under each wing – the XB-48 actually had only two, unusually wide, three-engined nacelles. Each of these large nacelles also contained an intricate set of air ducts that constituted the engines' cooling system.
The XB-48 was the first aircraft designed with bicycle-type tandem landing gear, which had previously been tested on a modified B-26. The wing airfoil was too thin to house conventional landing gear mechanisms. [2] The main landing gear was in the fuselage and small outriggers located on each wing were used to balance the aircraft.
During development, Boeing was using German swept-wing research to develop the dramatically more capable Boeing B-47 Stratojet for the same role. As this outperformed all of the Class of '45, development of the XB-48 continued only as a backup in case the B-47 ran into problems, which did not occur. The only member of the '47 to be put into production was the North American B-45 Tornado, which only served for a few years as a bomber before being replaced by the B-47.
Operational history
The XB-48 made its first flight on 22 June 1947, a 37-minute, 73 mi (117 km) hop from Martin's Baltimore, Maryland plant to NAS Patuxent River, Maryland, but blew all four tires on its fore-and-aft mounted undercarriage on landing when pilot Pat Tibbs applied heavy pressure to the specially designed, but very slow to respond, insensitive air-braking lever. Tibbs and co-pilot Dutch Gelvin were uninjured. [3]
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