Aft view of Martin XB-48. Note the tail armament rough-in. The large hemisphere was for the twin .50-cal. turret, and the smaller one was for the fire control radar antenna. Also note the Martin Mariner flying boat parked in the background (with the V-shaped twin tail). (U.S. Air Force photo)
Martin XB-48 (S/N 45-59586, the second XB-48 built) in flight with landing configuration. Note the "production" nose (no test probe) and the .50-cal. machine guns installed in the tail. (U.S. Air Force photo)
The Martin XB-48 (Martin Model 223) was the last of four designs (XB-45, XB-46, XB-47, XB-48) evaluated by the Army Air Force for a 1944 all-jet bomber requirement. The following description is from a June 1947 press release issued by the Glenn L. Martin Public Relations Department:
" ... the six-jet Martin XB-48, newest Army Air Forces high speed jet bomber made its initial flight Sunday (22 June 1947) from the Glenn L. Martin Company airport in Baltimore to the Patuxent River (Maryland) Naval Air Station, remaining aloft 37 minutes. Powered by six General Electric J-35 gas turbine engines housed three in each wing, the Martin XB-48 has a speed of over 480 miles per hour and carries a bomb load of more than 10 tons. It employs a new type "bicycle" landing gear, because of the difficulty of retracting heavy gears into extremely thin wings required for high speeds. Two pairs of main wheels are located tandem-style under the fuselage, and two smaller "outrigger" wheels farther out under the wing, to give stability during ground operations. The large main gear folds into the fuselage and the smaller wheels retract into the wings."
The tandem landing gear arrangement was initially tested on the Martin XB-26H, a modified B-26 with the basic tandem main landing gear with outriggers arrangement.
Because of the urgency of the jet bomber requirement, the two four-engine bombers competed against each other (XB-45 and XB-46) for an immediate production contract. The XB-45 won this competition. The XB-48 lost to the Boeing XB-47 in a performance fly-off of the two six-engine bombers. The XB-47 with its swept wing had much better performance. Only two prototype XB-48s were completed for flight testing before the entire program was canceled.
Six-engine jet bomber prototype
TECHNICAL NOTES: Armament: Designed for two .50-cal. machine guns in a radar-controlled tail turret and up to 22,000 lbs. of bombs (one 22,000-lb. "Grand Slam" or 14 1,000-lb. bombs maximum loading) Engines: Six Allison J35-A-5 axial flow turbojet engines of 4,000 lbs. thrust each maximum Maximum speed: 495 mph Cruising speed: 437 mph Range: 2,500 miles with 8,000 lbs. of bombs Service ceiling: 43,000 ft. Span: 108 ft. 4 in. Length: 85 ft. 8 in.
Height: 26 ft. 6 in. Weight: 102,600 lbs. (maximum takeoff weight) Crew: Three (pilot, copilot-radio operator-gunner, bombardier-navigator) Serial numbers: 45-59585 and 45-59586