The Martin XB-5
The Martin XB-5
The Martin XB-51 was an American trijet ground-attack aircraft. 0
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.[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.
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.
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.[3] 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/s2) 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.