Martin XB-48
Six-jet medium bomber prototype from the Glenn L. Martin Company, first flown on 22 June 1947. The XB-48 was one of the "Class of '45" designs drawn from the War Department's 1944 requirement: it paired six early, low-thrust J35 turbojets — three in each of two wide nacelles — with a straight-wing airframe in the B-26 Marauder tradition, and it was the first aircraft designed with bicycle-type tandem landing gear. Boeing's swept-wing B-47 outperformed it, development continued only as a backup, and the type was canceled in 1948 with two prototypes built and no production. Its landing-gear arrangement passed on to the B-47 and B-52.
Why It Mattered
The XB-48 was one of four designs drawn from the U.S. War Department's 1944 requirement for a new generation of medium bombers, a competition later nicknamed the "Class of '45". Its airframe belonged to the war just ended: the National Security Journal describes it as, in essence, a Second World War-era airframe fitted with jet engines. What it introduced was its undercarriage, for it was the first aircraft designed around a bicycle-type tandem landing gear. Boeing's swept-wing B-47 outperformed every member of the class, and Martin's work went on only as insurance in case the Boeing design stumbled. It never stumbled, so the insurance was never claimed, but the gear the XB-48 pioneered outlived the aircraft itself.
Design Notes
The fuselage was round in section and the wing unswept, a shape that showed a distinct influence from Martin's own B-26 Marauder. It took six of the era's low-thrust turbojets to power it. They sat not in six nacelles but in two: a single, unusually wide nacelle under each wing held three engines, with cooling air routed through an intricate system of ducts inside. The wing's airfoil was too thin to house a conventional landing gear, so the main gear was carried in the fuselage and small outriggers on each wing kept the aircraft balanced. That arrangement had been proven beforehand on a modified B-26, the XB-26H, which paired tandem main wheels with outrigger wheels in its engine nacelles. The crew was three — pilot, co-pilot and bomber-navigator — and the planned armament was a single 20,000 lb bomb or 36 250 lb bombs, with a tail turret of two .50 in machine guns that remained a proposal.
Operational History
Martin's 1947 announcement presented the type as the Army Air Forces' newest high-speed jet bomber, carrying a bomb load of more than ten tons. The first flight came on 22 June 1947, a 37-minute hop from Martin's Baltimore plant to the Naval Air Station at Patuxent River, Maryland. The landing went wrong: all four tires on the fore-and-aft undercarriage blew when pilot Pat Tibbs bore down on a specially designed air-braking lever that responded only slowly. Tibbs and his co-pilot, Dutch Gelvin, came through uninjured. Only two prototypes were ever built, and the type's remaining development ran as a backup against the chance that the B-47 would run into problems. Those problems did not occur; the XB-48 never entered production or active duty, and it is recorded as canceled in 1948. Of the class, only North American's B-45 Tornado was put into production, and it served a few years as a bomber before the B-47 replaced it.
Afterlife
The two prototypes, serial numbers 45-59585 and 45-59586, were the whole of the type; no production aircraft followed. Its bicycle undercarriage had the longer career: designed first for the XB-48, it was used on the Boeing B-47 Stratojet and, in modified form, on the B-52 Stratofortress. The B-47 it had insured against failure became the United States Air Force's main bomber for much of the 1950s and 1960s. The National Security Journal calls the type an airplane stuck between two ages, caught between the past and the future. That was true of its whole class, which combined turbojet power with the aeronautical design of the Second World War.