Staff Writer (Updated: 4/25/2016):
Inflight refueling was pioneered in the Cold War and streamlined with the arrival of the larger jet-powered aircraft. The result allowed military warplanners an unprecedented global reach when dictating actions of their forces. To this day, inflight refueling remains a large part of the successes of any one air force, especially when dealing with activities over large distances. It also takes a steady hand and eye for airmen and tanker crews alike to complete the precision process.
Design work began in the early 1950's to fulfill a USAF requirement for an inflight refueler with jet-powered capabilities. The result became the 367-80 demonstrator that furthermore became the KC-135. The KC-135 featured tanking equipment and an all-Boeing powered boom. Fuel was carried in the belly and wing elements and power was generated by use of four J57 powerplants. The system was generally crewed by three personnel though additions could be made when mission-specific (5 are utilized in the MEDEVAC role for instance).
Re-engining programs and system and software upgrades have propelled the KC-135 series into the new millennium. The system is still a large factor in global operations of the USAF and increases the reach of all aircraft on sortie. Other nations utilize the KC-135 as well, though it has not had quite the impact anywhere else as it has had for US forces traveling the globe.
Special thanks to Anthony S of the Boeing Corporation for his input.