# Martin XB-51

The Martin XB-51 was an American trijet ground-attack aircraft. It was designed in 1945 and made its maiden flight in 1949. It was originally designed as a bomber for the United States Army Air Forces under specification V-8237-1 and was designated XA-45. The "A" ground-attack classification was eliminated the next year, and the XB-51 designation was assigned instead. The requirement was for low-level bombing and close support. The XB-51 lost out in evaluation to the English Electric Canberra which — built by Martin — entered service as the Martin B-57 Canberra.

## Design and development

This unorthodox design, first flying on 28 October 1949, was fitted with three General Electric J47 engines - an unusual number for a combat aircraft - two underneath the forward fuselage in pods, and one at the extreme tail with the intake at the base of the tailfin. The innovative, variable incidence wings, swept at 35° and with 6° anhedral, were equipped with leading edge slats and full-width flaps. Spoilers gave most of the roll control and undersized ailerons provided feel for the pilot. The combination of variable incidence and slotted flaps gave a shorter takeoff run. 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.
The main landing gear consisted of dual wheel sets in tandem in the fuselage, similar to the Boeing B-47 Stratojet, with outrigger wheels at the wingtips (originally proved on a modified Martin B-26 Marauder named "Middle River Stump Jumper"). The XB-51 was a large but aerodynamically "clean" design which incorporated nearly all major systems internally. 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 required only a 4,000 lb (1,814 kg) bombload. Eight 20 mm cannon mounted in the nose would have been installed in production aircraft.
Crew was a pilot under a "fighter"-type bubble canopy and a Short-range navigation and bombing system (SHORAN) operator/navigator in a compartment located lower than and to the rear of the cockpit (only a small observation window was provided). Both crew members were provided with a pressurized, air conditioned environment, equipped with upward-firing ejection seats. The XB-51 was the first Martin aircraft equipped with ejection seats, these being of their own design.

## Operational history

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 Douglas A-26 Invader. The XB-51 was entered, as well as the Avro Canada CF-100 and English Electric Canberra; the XB-51 and Canberra emerged from these as the favorites.
Test flights showed the XB-51 to be highly maneuverable at low altitudes and substantially faster than the Canberra and faster than most fighter aircraft of the era. However, the XB-51's endurance was significantly lower than that of the Canberra and this factor was decisive in its cancellation. In addition, a load limiting factor of only 3.67 g (36 m/s$^2$) meant that the general strength of the airframe was relatively low and would prevent tight turns while fully loaded. Additionally, the tandem main gear plus outriggers of the XB-51 were thought unsuitable for the requirement to fly from emergency forward airfields.
While the XB-51 was not selected for procurement, it was decided that Martin would build 250 Canberras under license, under the designation B-57. Furthermore, Martin's rotating bomb bay would be incorporated into production variants of the B-57. A "Super Canberra", incorporating other XB-51 features, such as swept wings and tail-planes, was also proposed. This aircraft – although it promised much better speed and performance than the B-57 – never reached the prototype stage, mainly because the many changes would have taken too long to implement and test, before it could be put into production.
Flights by the XB-51 prototype, 46-685, continued, for general research purposes, following the project's official cancellation by the USAF. A second prototype, 46-686, which first flew in 1950, crashed during low-level aerobatics on 9 May 1952, killing pilot Major Neil H. Lathrop. 46-685 continued to fly, including an appearance in the film Toward the Unknown as the "Gilbert XF-120" fighter. 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.

## Specifications (XB-51)

Data from U.S.Standard Aircraft Characteristics
General characteristics
* Crew: 2
* Length: 85 ft 1 in (25.93 m)
* Wingspan: 53 ft 1 in (16.18 m)
* Height: 17 ft 4 in (5.28 m)
* Wing area: 548 sq ft (50.9 m$^2$)
* Airfoil: NACA 63A010
* Empty weight: 30,906 lb (14,019 kg)
* Gross weight: 57,874 lb (26,251 kg)
* Max takeoff weight: 62,452 lb (28,328 kg)
* Powerplant: 3 × General Electric J47-GE-13 turbojet engines, 5,200 lbf (23 kN) thrust each
Performance
* Maximum speed: 644 mph (1,036 km/h, 560 kn)
* Range: 1,075 mi (1,730 km, 934 nmi)
* Ferry range: 1,444 mi (2,324 km, 1,255 nmi)
* Service ceiling: 41,750 ft (12,730 m)
* Rate of climb: 6,600 ft/min (34 m/s)
* Wing loading: 105.6 lb/sq ft (516 kg/m$^2$)
* Thrust/weight: 0.27
Armament
* Guns: 8 × 20 mm M24 cannon (0.79 in) cannon with 1,280 rounds
* Rockets: 8 × High Velocity Aerial Rockets (HVAR) or
* Bombs: Up to 10,400 lb (4,720 kg) carried internally