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Glenn L. Martin

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Martin Models 223 and 247

XB-48 Jet Bomber
Variants/Specifications

After issuing specifications for the first generation of jet bombers in April 1944, the Army Air Forces ordered competing prototypes from four companies. It was inconceivable that there would not be a Martin Bomber among them; the other manufacturers were North American, Consolidated, and Boeing. The whole series was designated XB-45 through 48. (Later on the Northrop flying wing, re-engined with jets, also joined the competition as the YB-49.) All of the designs were to use Allison/General Electric) J35 engines with 4,000 pounds of thrust The North American and Consolidated designs each mounted four engines, the Boeing and Martin ones six. All were to be capable of carrying the 22,000-pound "Grand Slam" bomb (a stand-in for the still secret atomic bomb) and crews of two pilots and a navigator/bombardier. Bombers flying at 500 mph, it was felt, did not need defensive gunners, only radar-controlled tail turrets.

North American led the race with the medium-range B-45, which was ordered into production in January 1947. The bigger six-jet bombers took longer. The AAF dealt a winning technological hand to Boeing's designers, who were encouraged to incorporate into their XB-47 the swept wings and pod mountings found more successful by German jet pioneers. Martin engineers were told to stick with a "conventional design" of straight wings and engines in wing nacelles. Ken Ebel, by this time Martin's Vice President in charge of engineering, nevertheless tried several innovations in the Martin XB-48. Its two huge nacelles, each housing three jet engines, were themselves designed as lifting bodies; internal air ducts between the engines were supposed to channel the airstream. The wings were extremely thin for low drag at high speed. This trick was accomplished by a "bicycle" landing gear that folded into the fuselage instead of the wing; it was tested in 1946 on the converted XB-26H Middle River Stump Jumper.

In order to speed the construction of the prototype, Ebel and Tom Willey (now returned from Martin-Nebraska) established a "company within a company," moving a picked engineering staff to the B Building floor alongside the mockup, with its own tool designers, machinists, bookkeepers, and stores department. In the interests of speed, needed parts would be made quickly on the spot, without regard to specified weights, materials, or detailed drawings. Records were kept, however, so that they could be properly replaced in the next prototype.

The first XB-48 made its first flight on June 22, 1947, a 37-minute hop to Patuxent River Naval Air Station, where the 11,000-foot runway offered a larger margin for safety than the 7,100-foot factory strip. Accustomed as they were by this time to test flights, the plant's neighbors were taken aback by the smoky plumes of the six jet engines and called police with reports of an airplane on fire.

Tests at Patuxent River were disappointing. The XB-48's top speed was only 516 mph at 20,000 feet, 479 at the design altitude of 35,000. Test pilot E.R. "Dutch" Gelvin reported that the ducted nacelles, which had worked well enough in the wind tunnel, dammed the airflow at high speeds. The Boeing XB-47, tested the following year in California, was the clear winner. Its top speed at 15,000 feet was 580 mph, 545 at 35,000, and its range was greater.

Boeing received the first production order for the B-47 in September 1948. In October the second prototype Martin XB-48, its weight duly brought down to specification first flew. It was used to test equipment for its successful competitor. Kept in repair with parts taken from the first prototype, it was equipped with high-altitude de-icing blankets and other devices. When these tests ended in mid 1951, the XB-48 was flown to Aberdeen Proving Grounds and destroyed in static structural tests.

In 1949, Glenn Martin invited Air Force officials to redeem some of their $11.5 million investment in the plane (and his company's sagging fortunes) by ordering the Martin Model 247, a turboprop-powered version of the XB-48 airframe. Unlike the B-47, this would have true intercontinental range. The Air Force saw its future in jets, however, and turned the proposal down.



XB-48


XB-48


© 2006 The Glenn L. Martin Maryland Aviation Museum
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