Martin XB-48 prototype taxiing, showing spaces between engines for cooling, tandem main gear, & nacelle outriggers.
The Martin XB-48 was a medium jet bomber developed in the mid-1940s. It competed with the Boeing B-47 Stratojet, a clearly superior design, and was seen largely as a backup plan in case the B-47 ran into development problems. It never saw production or active duty, and only two prototypes, serial numbers 45-59585 and 45-59586, were built.
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 lb (36,287 kg) to more than 200,000 lb (90,718 kg). Other designs resulting from this competition, sometimes named the class of '45, included the North American XB-45 and the Convair XB-46. Production orders finally went to the North American B-45 Tornado, and even this airplane only served for a couple of years before again being replaced by the much more modern Boeing B-47 Stratojet although the B-45 had enough "utility" built in to maintain a niche as reconnaissance aircraft.
In retrospect, the class of '45 were transitional aircraft combining the power of turbojets with the aeronautical knowledge of World War II. The XB-48 was no exception, as its round fuselage and unswept wings show a distinct influence of Martin's B-26 Marauder medium bomber. Still, where the B-26 had enough thrust with two massive 18-cylinder radial engines, the XB-48 needed no less than six of the new jet engines.
Although the pictures make it look like the aircraft has three engine gondolas under each wing, the jet engines were actually clustered in a pair of flat three-engined gondolas with an intricate system of air canals between the engines providing cooling. At the time of the XB-48's design, jet propulsion was clearly still in its infancy.
The XB-48 was the first aircraft designed with bicycle type landing gear, which had previously been tested on a modified B-26. The wing airfoil was too thin to house conventional landing gear mechanisms. The main landing gear was in the fuselage and small outriggers located on each wing were used to balance the aircraft.
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.
^Knaack, Marcelle Size. Encyclopedia of U.S. Air Force Aircraft and Missile Systems: Volume II: Post-World War II Bombers, 1945–1973. Washington, DC: Office of Air Force History, 1988. ISBN 0-16-002260-6.
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.
Mizrahi, Joe. "The Last Great Bomber Fly Off". Wings, Volume 29, Number 3, June 1999.