The C-135A (Boeing 717-157) was bought by the USAF as an interim jet transport while studies into what would become the C-141 StarLifter were carried out. Initially 50 aircraft were approved but ultimately only fifteen were ordered, along with the conversion of three KC-135As on the production line. Thirty C-135Bs (Boeing 717-158) made up the balance, being near identical but with TF33 engines rather than J57s. The C-135 featured the deletion of the air refuelling system, a strengthened cargo floor and the addition of a fuel dumping capability. Although the C-135 offered much over the piston and turboprop transports then in service it was less flexible due to the side loading cargo door, while the C-135A required water injection and a long runway when fully loaded, curtailing its usefulness. In addition it only had an endurance of five-six hours, fine for crossing the Atlantic but not the Pacific. Ultimately Secretary of Defense Robert McNamara directed the USAF to find other uses for them once the C-141 was available.
And they certainly did. All C-135As were converted into different configurations, including KC-135A-II tankers, JC-135A nuclear test support aircraft, and EC-135N Apollo Range Instrumented Aircraft (ARIA). C-135Bs lasted longer as transports, though many of them were converted as test range telemetry aircraft, WC-135B weather reconnaissance aircraft, VC-135B VIP transports or assorted RC-135s. The USAF certainly had its money's worth.
All info c/o The Boeing KC-135 Stratotanker- More Than A Tanker (Robert S. Hopkins III), Crecy Publishing 2017