Martin 316

The Martin Model 316 (military designation XB-68) was envisioned as a supersonic medium tactical bomber with a crew of two for the United States Air Force.

Design and development
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, which was designated the B-66 Destroyer. 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 (122.3 kN) static sea level thrust each with afterburner, providing a maximum speed of 1,588 mph (1,380 kn, 2,556 km/h) at 54,700 ft (16,673 m) altitude at maximum power and a combat speed of 1,534 mph (1,333 kn, 2,469 km/h) at 42,200 ft (12,863 m) altitude at maximum power. Combat range was planned for 1,250 mi (1,086 nmi, 2,011 km) with 3,700 lb (1,680 kg) payload at 526 kn (974 km/h) average in 4.15 hours.