[3] In the early 1960s, the Military Air Transport Service operated a fleet consisting almost entirely of propeller-driven aircraft such as the piston-powered Douglas C-124 Globemaster II and C-133 Cargomaster turboprop.

[5][6] While capable of carrying large, outsized payloads, they were becoming increasingly obsolescent and lacked the global reach required of the rapidly-modernizing Air Force.

In May 1960, Congress approved the purchase of 50 C-135 aircraft; it was selected in part because of its low development cost, being a straightforward derivative of the KC-135 tanker already in production.

The C-135 was largely intended as an interim measure pending the development of more specialized jet transports such as the Lockheed C-141 Starlifter, and as such it incorporated numerous compromises in its strategic airlift capability.

The aircraft's load floor was some 10 feet (3.0 m) off the ground, which required ground-handling equipment, its single side-loading cargo door was limited in what could fit through it, and its useful range was approximately 6,000 miles (9,700 km), insufficient to reach many of the Air Force's operating locations in Asia and the Pacific Rim.

Additionally, its takeoff and landing performance required long runways available only at the largest military bases or commercial airports, which were not necessarily located in close proximity to potential combat areas.

Ten were modified for a weather reconnaissance (flying through radioactive clouds from nuclear tests or other agents) role and designated WC-135B Stratolifter (Constant Phoenix in later versions).

Developmental tests using this aircraft have demonstrated the capability to fly precision approaches using a local area differential GPS system.

It was also the official nickname given to a modified C-135C, serial number 61–2669, that was used by the Secretary and the Chief of Staff of the Air Force for executive transport requirements.