The Boeing C-135 family, developed from the 367-80 commercial jet testbed that also spawned the Boeing 707 line, has served the US Air Force outstandingly well since 1957. The USAF received its first KC-135A tanker that year, and since then the type has provided air-to-air refuelling support the world over, acting as a key component of combat operations and enabling the air arm's 'global reach'. A total of 732 Stratotankers were built, and while those still in service today were built, in some cases, as far back as the late 1950s they have been subject to much in the way of upgrading. A re-engining programme, using the more fuel-efficient CFM56 turbofan powerplant, produced the KC-135R model as flown today by active-duty USAF units. These include the 100th Air Refueling Wing's 351st Air Refueling Squadron, the sole USAF tanker unit permanently based in Europe, at RAF Mildenhall in Suffolk. While Boeing's KC-46A Pegasus, a tanker development of the 767 airliner, has been procured as the USAF's next-generation aerial refueller, the KC-135 still has a long service life ahead of it.