By William Pearce

In February 1946, the United States Army Air Force (AAF) sought design proposals for an attack aircraft to replace the Douglas A-26 Invader. The Glenn L. Martin Company (Martin) responded with its Model 234, a straight-wing aircraft of a rather conventional layout, except that the engine nacelle on each wing housed a turboprop and a turbojet engine. The Model 234 had a crew of six and was forecasted to carry 8,000 lb (3,629 kg) of ordinance over 800 miles (1,287 km).

Martin was awarded a contract to develop the Model 234 on 23 May 1946, and the aircraft was designated XA-45. A few weeks later, the AAF decided to discard the “Attack” category, and the XA-45 was subsequently redesignated XB-51. The AAF then requested new requirements for the XB-51 with an emphasis on speed. The AAF’s new desired specifications for the A-26 replacement was a top speed of 640 mph (1,030 km/h) and the ability to carry 4,000 lb (1,814 kg) of ordinance over 600 miles (966 km). The new requirements necessitated a complete redesign of the XB-51, which Martin completed and submitted to the AAF in February 1947. After slight modifications, the design was somewhat finalized by July 1947. The AAF ordered two prototypes, which were assigned serial numbers 46-685 and 46-686.

The Martin XB-51 was a radical departure from the firm’s previous aircraft designs. The XB-51 was an all-metal aircraft that featured a relatively large fuselage supported by relatively small swept wings. The aircraft had a crew of two and was powered by three General Electric J47-GE-13 engines, each developing 5,200 lbf (23.13 kN) of thrust. Two of the engines were mounted on short pylons attached to the lower sides of the aircraft in front of the wings. The third engine was buried in the extreme rear of the fuselage.

The pilot sat in the front of the aircraft under what appeared to be a small canopy in contrast to the large fuselage. Behind the pilot and completely within the fuselage was the radio operator, who was also in charge of the short range navigation and bombing (SHORAN) system. The crew compartment was pressurized, and access was provided by a door on the left underside of the fuselage, between the pilot and radio operator’s stations. In case of an emergency, both crew were provided with upward firing ejection seats.

The engine housed in the rear fuselage was fed by an inlet duct located atop the fuselage. A rotating assembly was installed forward of the inlet to either cover the inlet with an aerodynamic fairing when the engine was not in use, or rotate to provide a duct to feed air to the engine. The rear engine could be shut down in flight to extend the aircraft’s range. When not in use, a door in the intake duct prevented the back flow of air through the rear engine. Large doors swung open beneath the fuselage to access the rear engine.

Mounted above the rear engine was the vertical stabilizer, with the horizontal stabilizer mounted to its top. Originally, the XB-51’s design had the horizontal stabilizer mounted midway up the vertical stabilizer, but the aircraft was not built with this configuration. The horizontal stabilizer was swept back 35 degrees, and its incidence could be changed for trimming. Two rocket assisted takeoff (RATO) bottles could be fitted to each side of the rear fuselage. The RATO packs would be ignited to shorten the XB-51’s takeoff distance, then discarded once the aircraft was in flight. Each bottle provided 1,000 lbf (4.44 kN) of thrust. Hydraulically operated air brakes were located on each side of the fuselage, under the intake for the rear engine. A braking parachute was housed in the left side of a fairing located below the rudder.

The XB-51 used tandem (bicycle) main gear that consisted of front and aft trucks, and outrigger wheels that deployed from the aircraft’s wingtips for support. Martin had used a similar gear arrangement for the straight-wing XB-48 jet medium bomber and had initially tested the setup using the Martin XB-26H, a B-26 Marauder specially modified for to test the tandem landing gear. The main trucks could swivel to counteract the aircraft’s yaw while taking off or landing with a crosswind.

The aircraft’s bomb bay was located in the fuselage between the main wheels. The bomb bay had a single rotating door to which the bomb load was attached. Opening the rotating door did not create any buffeting or require any speed restriction normally required by two conventional doors. In addition, the rotating door was removable and could be quickly replaced with another door already loaded with ordinance. The standard door could accommodate nine 500 lb (227 kg) bombs, four 1,000 lb (454 kg) bombs; four 1,600 lb (726 kg) bombs; or two 2,000 lb (907 kg) bombs. Two additional 2,000 lb (907 kg) bombs could be accommodated on exterior bomb racks mounted on the bottom of the door. A special enlarged door could be fitted to carry a single 4,000 lb (1,814 kg) bomb or a Mk 5 or Mk 7 nuclear bomb. The XB-51’s maximum bomb load was 10,400 lb (4,717 kg). Eight 5 in (127 mm) High Velocity Aircraft Rockets (HAVR) could be carried in the bomb bay in place of internal bombs.

Three fuel tanks were installed in the aircraft’s fuselage. The forward tank was located above the front main gear and held 640 US gal (2,426 L). The center and aft tanks were both located above the bomb bay and held 745 US gal (2,820 L) and 1,450 US gal (5,489 L) respectively. All the standard fuel tanks could be filled via a single fueling receptacle. A 160.5 US gal (607.6 L) water/alcohol tank to boost engine performance during takeoff was mounted between the front and center fuel tanks. Two 350 US gal (1,325 L) tanks could be carried in the bomb bay for ferrying the aircraft over long distances. The XB-51 had a total normal fuel capacity of 2,835 US gal (10,732 L), and 3,535 US gal (13,381 L) with the bomb bay tanks.

In the nose of the XB-51 were eight fixed 20 mm cannons with 160 rpg and a forward strike camera. The nose of the second XB-51 was detachable, and different noses could be fitted depending on the aircraft’s mission. In addition to the standard gun nose, other noses featured equipment for precision bombing and equipment for photo-reconnaissance. As standard, the XB-51 had a reconnaissance camera installed under the cockpit and a strike assessment camera installed in the lower rear fuselage.

With fuel, engines, and the main landing gear all housed in the fuselage, the XB-51’s mid-mounted wings were very thin. The wings were swept back 35 degrees and had six degrees of anhedral. Outrigger wheels deployed from the wingtips to steady the aircraft on the tandem main gear. Slats extended along the outer 70 percent of the wing’s leading edge. Large, slotted flaps covered 75 percent of the wings trailing edge, with small ailerons taking up 15 percent of the trailing edge. While the ailerons contributed to aircraft’s roll control, their main purpose was to provide feedback for the pilot. The majority of roll control was provided by spoilers positioned on the wing’s upper surface, just forward of the flaps. The spoilers extended about 40 percent of the wing’s span. The incidence of the entire wing could vary from 2 to 7.5 degrees and would automatically change with deployment of the flaps. The wing incidence increased at lower speeds to decrease the aircraft’s stall speed and make the aircraft assume the correct attitude for landing, which was with the nose high approximately six degrees. The tandem landing gear required the simultaneous touchdown of both the forward and aft trucks. To prevent the accumulation of ice, hot air was bled off from the engines, directed through a passageway in the wing’s leading edge, and exhausted out the wingtip.

The Martin XB-51 had a 53 ft 1 in (16.18 m) wingspan, was 85 ft 1 in (25.93 m) long, and was 17 ft 4 in (5.28 m) tall. The track between the outrigger landing gear was 49 ft 5 in (15.06 m). The aircraft had a top speed of 645 mph (1,038 km/h) at sea level and 580 mph (933 km/h) at 35,000 ft (10,668 m). Cruising speed was 532 mph (856 mph) at 35,000 ft (10,668 m), and the aircraft’s landing speed was around 140 mph (225 km/h). The XB-51’s initial rate of climb was 6,980 ft (35.5 m/s) at maximum power and 3,600 ft (18.3 m/s) at normal power. The service ceiling was 40,500 ft (12,344 m); normal range was 980 miles (1,577 km), and ferry range was 1,445 miles (2,326 km). The XB-51 had an empty weight of 30,906 lb (14,019 kg), a combat weight of 44,000 (19,958 kg), and a gross weight of 55,930 lb (25,369 kg).

On 24 February 1948, a mockup of the XB-51 was inspected by the United States Air Force (USAF), which had become a separate branch of the US Armed Forces on 18 September 1947. Construction of the first prototype (46-685) proceeded swiftly at the Martin plant in Middle River, Maryland, and the completed aircraft was rolled out on 4 September 1949. After completing ground tests, aircraft 46-685 made is first flight on 28 October 1949, piloted by Orville Edward ‘Pat’ Tibbs. Initial flight testing went well until the rear main gear collapsed after landing on 28 December. The aircraft was repaired and returned to flight status in early 1950. High-speed testing had revealed some vibrations with the tail and a tendency to Dutch roll. A bullet faring was added at the intersection of the horizontal and vertical stabilizers in March 1950 to mitigate the issues.

The second prototype (46-686) made its first flight on 17 April 1950, piloted by Frank Earl ‘Chris’ Christofferson. Although 46-686 was initially flown with the original tail, bullet fairings were soon added. Both aircraft were involved in numerous landing accidents, mostly attributed to the tandem landing gear and the pilot’s lack of familiarity with its nuances. Nose high landings resulted in tail strikes that damaged the aft fuselage. Nose low and hard landings resulted in the collapse or shearing of the front main gear. Despite the landing difficulties, pilots seemed to like the aircraft and its performance. While the XB-51 could perform rolls and outpace some fighters, the aircraft was not stressed for aggressive maneuvers.

The USAF considered putting the XB-51 into production, but the role for which the aircraft was intended had changed again with the outbreak of the Korean War. Speed was no longer the main focus, and the USAF now desired an aircraft that could loiter in an area until needed by ground forces. The USAF compared the XB-51 against the North American AJ-1 Savage and B-45 Tornado, the Avro Canada CF-100 Canuck, and the English Electric Canberra. Under the new criteria, the USAF selected the Canberra as the winner in February 1951, and the XB-51 program was essentially cancelled. The Canberra had more than twice the range and loiter time of the XB-51. The following month, Martin was awarded a contract to build the Canberra as the B-57, and the rotary-style bomb bay pioneered on the XB-51 was installed on the B-57. Ultimately, 403 B-57 aircraft would be produced. Both XB-51 aircraft continued to be evaluated and tested. The two XB-51s underwent performance and armament tests at Edwards Air Force Base (AFB) in California and Elgin AFB in Florida.

On 9 May 1952, the second prototype XB-51 was destroyed at Edwards AFB when Major Neal Lathrop executed a roll at low altitude and collided with the ground. Lathrop was the sole occupant on board. At the time of the accident, 46-686 had accumulated 151 hours of flight time and had made 193 flights.

The first prototype played the role of the “Gilbert XF-120” fighter in the 1956 movie “Toward the Unknown.” The movie was shot mostly at Edwards AFB in 1955. On 25 March 1956 the 46-685 was destroyed while taking off from El Paso Municipal (now International) Airport in Texas. The stop in El Paso was to refuel as the aircraft traveled from Edwards AFB to Eglin AFB. The accident occurred due to a premature rotation and subsequent stall. The radio operator, Staff Sergeant Wilbur R. Savage, was killed in the crash, and the pilot, Major James O. Rudolph, died of his injuries on 16 April 1956. The first XB-51 prototype had accumulated 432 hours and made 453 flights.

Performance of the Martin XB-51 had exceeded the manufacturer’s guarantees. However, the aircraft was designed and built at a time when USAF’s desires and priorities were rapidly shifting, and it turned out that the service did not really want the aircraft they had originally asked for. Pilots held the XB-51 in a high regard despite its demanding landing characteristics. Ultimately, the XB-51 faded into history as a short-lived experimental aircraft investigating a new direction at the dawn of the jet age.

Sources:
The Martin XB-51 by Scott Libis (1998)
“Martin XB-51” by Clive Richards, Wings of Fame Volume 14 (1999)
Martin Aircraft 1909–1950 by John R. Breihan, Stan Piet, and Roger S. Mason (1995)
Standard Aircraft Characteristics XB-51 by U.S. Air Force (11 July 1952)
Jane’s All the World’s Aircraft 1951-1952 by Leonard Bridgman (1951)
U.S. Bombers 1928 to 1980s by Lloyd S. Jones (1980)
https://www.avgeekery.com/the-xb-51-martins-movie-star-bomber-smashed-the-bomber-mold/
https://www.thisdayinaviation.com/tag/martin-xb-51/