Design and development
Both prototypes taking off in formation
Testing RATO
The resulting unorthodox design, first flying on 28 October 1949, was (unusually for a combat aircraft) fitted with three engines, General Electric J47s in this case: one at the extreme tail with an intake at the base of the tailfin, and two underneath the forward fuselage in pods.[2] The innovative wings, swept at 35° and with 6° anhedral, were equipped with variable incidence, leading-edge slats, full-width flaps and spoilers instead of ailerons. The combination of variable incidence adjustment and slotted flaps allowed for a shorter takeoff run.[3] Four 954 lb (4.24 kN) thrust Rocket-Assisted Take Off (RATO) bottles with a 14-second burn duration could be fitted to the rear fuselage to improve takeoff performance. Spectacular launches were a feature of later test flights.[2]
The main landing gear consisted of dual sets of wheels in tandem in the fuselage, similar to the B-47 Stratojet, with outrigger wheels at the wingtips (originally proved on a modified B-26 Marauder named "Middle River Stump Jumper"[2]). The B-51 was a large but aerodynamically "clean" design which incorporated nearly all major systems internally.[3] The aircraft was fitted with a rotating bomb bay, a Martin trademark; bombs could also be carried externally up to a maximum load of 10,400 lb (4,700 kg), although the specified basic mission only required a 4,000 lb (1,814 kg) bombload.[4] Eight 20 mm (.79 in) cannons mounted in the nose would have been installed in production aircraft.[3]
Crew provision was for a pilot under a "fighter"-type bubble canopy and a SHORAN (short-range navigation and bombing system) operator/navigator in a compartment located lower than and to the rear of the cockpit (only a small observation window was provided).[3] Both crew members were provided with a pressurized, air-conditioned environment, equipped with upward-firing ejection seats.[3] The XB-51 was the first Martin aircraft equipped with ejection seats; the ejection seats being of their own design.[5]
Operational history
A shot of 46-685 on approach from the archives of the National Museum of the U.S. Air Force
In 1950, the United States Air Force issued a new requirement based on early Korean war experience for a night intruder/bomber to replace the A-26 Invader. The XB-51 was entered, as well as the Avro Canada CF-100 and the English Electric Canberra. The Canberra and XB-51 emerged as the favorites. The XB-51 was a highly maneuverable aircraft at low level, and substantially faster than the Canberra (its "turn-of-speed" was faster than most fighter aircraft of the era[3]). However, its load limiting factor of only 3.67 g (36 m/s2) restricted tight turns, and the XB-51's endurance was substantially poorer than the Canberra's; this latter proved to be the deciding factor. Additionally, the tandem main gear plus outriggers of the XB-51 was thought unsuitable for the requirement to fly from emergency forward airfields.
The Canberra was selected for procurement and the XB-51 program ended. Martin did not end up the loser, however, for they were selected to build the 250 Canberras ordered under the designation B-57A. Furthermore, the rotating bomb bay was incorporated in the B-57. A proposed B-57 Super-Canberra also included XB-51 features, such as swept wing and tailplane. In the end it was never built, mainly because it was a new design and would have taken too long to put in production, although it promised much better speed and performance.[2]
Flight testing for research purposes continued after program cancellation. The second prototype, 46-686, which first flew in 1950, crashed on 9 May 1952 during low-level aerobatics. The first prototype, 46-685 continued to fly, including appearing in the film Toward the Unknown as the "Gilbert XF-120" fighter.[6] The surviving prototype was en route to Eglin AFB to shoot additional footage when it crashed during takeoff following a refueling stop in El Paso, Texas, on 25 March 1956.[3]
Specifications (XB-51)l characteristics
Crew: 2
Length: 85 ft 1 in (25.9 m)
Wingspan: 53 ft 1 in (16.2 m)
Height: 17 ft 4 in (5.3 m)
Wing area: 548 ft² (50.9 m²)
Empty weight: 29,584 lb (13,419 kg)
Loaded weight: 55,923 lb (25,366 kg)
Max. takeoff weight: 62,457 lb (28,330 kg)
Powerplant: 3 × General Electric J47-GE-13 turbojets
Performance
Maximum speed: 645 mph (1,040 km/h)
Range: 1,075 mi (1,730 km)
Ferry range: 1,613 mi (2,596 km)
Service ceiling: 40,500 ft (12,300 m)
Rate of climb: 6,980 ft/min (35.5 m/s)
Wing loading: 102 lb/ft² (498 kg/m²)
Thrust/weight: 0.28
Armament
Guns: 8 × 20 mm (0.79 in) cannon with 1,280 rounds
Rockets: 8 × High Velocity Aerial Rockets (HVAR)
Bombs: 2,000 (907 kg)