Martin XB-68

The XB-68 was envisioned as a supersonic medium tactical bomber with a crew of two for the United States Air Force. The Glenn L. Martin Company submitted design studies in response to the Weapon System 302A requirement in 1952 in competition with proposals from Douglas Aircraft Company and North American Aviation, Inc. Revised designs were presented again in 1954. The Boeing Airplane Company also submitted a design after the competition date had passed and was automatically rejected. Martin was declared the winner in 1956 and the B-68 designation applied to their design. Deployment was projected for the 1962-1965 period.

An orthodox layout that resembled somewhat a scaled-up Lockheed F-104, the XB-68 was to have been primarily of steel construction, with the crew of a pilot-radio operator and navigator-bombardier defense systems operator in a pressurized compartment to be cooled by filtered bleed-air from the engines, and a refrigeration unit for evaporative cooling at high Mach numbers. The B-68 would have had stubby diamond-shaped wings and a raked T-tail empennage. It was intended to be operated at supersonic speeds at medium and high altitudes.

The design immediately ran into serious difficulties over the inertial guidance bombing and navigation system, which, had the bomber been approved for production, would have pushed deployment back to at least 1963. The problems were rendered moot when Air Force Headquarters cancelled the project in 1957 citing stringent budget limitations and higher priorities on other weapon systems. Recognizing that the medium tactical bomber design was still years away, plans were carried forward instead to continue use of an Air Force version of the Navy's Douglas A3D, that received the designation B-66. Two XB-68 prototypes and one static test model were cancelled and none were built.

Planned power was two Pratt & Whitney J75 (JT4B-21) axial flow turbojets of 27,500 lbf (122,000 N) static sea level thrust each with afterburner, providing a maximum speed of 1380 knots at 54,700 feet altitude at maximum power and a combat speed of 1333 knots at 42,200 feet altitude at maximum power. Combat range was planned for 1,250 statute miles (1086 nautical miles or 2011 kilometers) with 3700 lb (1680 kg) payload at 526 knots (974 km/h) average in 4.15 hours.

The B-68 designation was subsequently applied to Martin's Titan intercontinental ballistic missile (as SM-68, redesignated HGM-25 in 1962).

Specifications (as designed)

General characteristics

• Crew: 2: pilot and bombardier/navigator
• Length: 109 ft 8 in (33.43 m)
• Wingspan: 53 ft 0 in (16.2 m)
• Height: 25 ft 6 in (7.77 m)
• Wing area: 875 ft² (81.3 m²)
• Empty weight: 53,925 lb (24,460 kg)
• Loaded weight: 74,180 lb (33,650 kg)
• Max takeoff weight: 102,720 lb (46,590 kg)
• Powerplant: 2× Pratt & Whitney JT4B-21 turbojets, 27,500 lbf (122 kN) each

Performance

• Maximum speed: 1,593 mph (2,564 km/h)
• Range: 3,051 mi (4,910 km)
• Service ceiling: 44,800 ft (13,650 m)
• Rate of climb: 500 ft/min (25 m/s)
• Wing loading: 85 lb/ft² (414 kg/m²)
• Thrust/weight: 0.74

Armament

• Guns: 1× 20 mm (0.787 in) M61 Vulcan rotary cannon with 1,100 rounds in tail
• Bombs:
  
  • 1× Class C nuclear bomb or
  • 1× Class D nuclear bomb

References

• Andrade, John M. (1979). U.S. Military Aircraft Designations and Serials since 1909. Earl Shilton, Leicester: Midland Counties Publications. ISBN 0-904597-22-9. 
• Knaack, Marcelle Size (1988). Encyclopedia of U.S. Air Force Aircraft and Missile Systems, Volume II - Post-World War II Bombers 1945-1973. Washington, D.C.: Office of Air Force History, USAF. ISBN 0-912799-59-5. 
• Wagner, Ray (1968). American Combat Planes (2nd ed.). Garden City: Doubleday and Company. ISBN 0-385-04134-9. 

Related content

Designation sequence

SM-65 - B-66 - GAM-67 -	XB-68	-	RB-69	-	XB-70	-	SR-71
	SM-68 I		IM-69		IM-70		GAM-71
	SM-68 II

Related lists

• List of bomber aircraft
• List of military aircraft of the United States