World War 2 (1939-1945) saw the rise of turbojet technology for future military aircraft applications. It became known to the Allies that the Germans were hard at work on potential war-winning jet-powered designs so, to stay in step with the enemy, both the British and the Americans enacted considerable jet aircraft programs of their own. In 1944, American authorities put forth a requirement calling for a jet-powered, medium-class bomber and this led to the emergence of several notable designs of the wartime period.
During this time, the Glen L. Martin Company, a long-time "big aircraft" developer for the United States air service, decided to try its hand at fulfilling the new request. This endeavor became its "XB-48" entry and two prototypes were eventually realized for all of the investment. The design largely followed the wartime forms that Martin was closely associated with, the primary difference being powerplants formed of turbojet technology.
Martin engineers elected for a tubular, streamlined fuselage with a glazed nose section and elevated flight deck slightly aft. The straight wing mainplanes were mid-mounted along the fuselage sides and placed near midships. The tail unit incorporated a single vertical fin with slightly upward-canted horizontal planes. A bicycle undercarriage featuring an inline twin leg arrangement was used with underwing assistance coming from smaller outboard support legs (the XB-48 was the first aircraft to ever feature a "bicycle" undercarriage arrangement). The aircraft would be crewed by at least three, made up of two pilots and a bomber/navigator, and power was to come from 6 x General Electric J35 (Allison J35) turbojet engines developing 3,820 pounds of thrust each. The jets would be fitted into the wing mainplanes as three per wing and a specially-developed, three-engined gondola was devised to help channel airflow around the engines in flight. This assembly gave the external impression of three individual engine nacelles being fitted but this was not the case. The J35 engine became the first axial-flow compressor engine for the United States Army Air Force / Air Force service and eventually powered the later Republic F-84 Thunderjet and Northrop F-89 Scorpion lines.
Dimensions included a length of 26 meters, a wingspan of 33 meters and a height of 8 meters. Armament was headed by an internal load of up to 20,000 pounds of conventional drop stores. Local defense would be provided by 2 x 12.7mm M7 heavy machine guns fitted to a tail turret (this was never fitted).
A first flight was recorded on June 22nd, 1947 and lasted less than an hour. The venture proved a short-lived success as the landing saw all of the undercarriage wheels blow out. However the crew (of two test pilots) were unhurt and the aircraft suffered no catastrophic damage. The test phase became a short one when the USAF cancelled development work on the XB-48. By this time, the grand world war had been over for some years and a massive military drawdown ensued, leaving many designs like the XB-48 out in the lurch. Things for the Martin design were made worse when the USAF focused on the jet-powered, swept-wing Boeing B-47 "Stratojet" bomber instead, the XB-48 essentially acting as insurance should the Boeing program suffer any developmental setbacks. The B-47 also flew for the first time in 1947 and was eventually adopted for service in 1951 - some 2,032 production examples following. When the B-47 proved a surer deal, there left little room for the XB-48 going forward.