The KC-135 was initially purchased to support bombers of the Strategic Air Command, but by the late 1960s, in the Southeast Asia theater, the KC-135 Stratotanker's ability as a force multiplier came to the fore. Midair refueling of F-105 and F-4 fighter-bombers as well as B-52 bombers brought far-flung bombing targets within reach, and allowed fighter missions to spend hours at the front, rather than just a few minutes, due to their limited fuel reserves. KC-135 crews refueled both Air Force and Navy / Marine Corps aircraft, though they would have to change to probe and drogue adapters depending upon the mission. Crews also helped to bring in damaged aircraft which could fly while being fed by fuel to a landing site or to ditch over the water. KC-135s continued their tactical support role in later conflicts such as Desert Storm and current aerial strategy.
The Strategic Air Command (SAC) had the KC-135 Stratotanker in service with Regular Air Force SAC units from 1957 through 1992 and with SAC-gained Air National Guard (ANG) and Air Force Reserve (AFRES) units from 1975 through 1992. Following a major USAF reorganization that resulted in the inactivation of SAC in 1992, most KC-135s were reassigned to the newly created Air Mobility Command (AMC). While AMC gained the preponderance of the aerial refueling mission, a small number of KC-135s were also assigned directly to United States Air Forces in Europe (USAFE), Pacific Air Forces (PACAF) and the Air Education and Training Command (AETC). All Air Force Reserve Command (AFRC) KC-135s and most of the Air National Guard (ANG) KC-135 fleet became operationally-gained by AMC, while Alaska Air National Guard and Hawaii Air National Guard KC-135s became operationally-gained by PACAF.
Air Mobility Command (AMC) manages 414 Stratotankers, of which the Air Force Reserve Command (AFRC) and Air National Guard (ANG) fly 247 in support of AMC's mission as of May 2014.
The KC-135 is joined by the Tupolev Tu-95, the C-130 Hercules, the B-52 Stratofortress, the English Electric Canberra, the Northrop T-38 Talon and the Lockheed U-2 in having over 50 years of continuous service with its original operator.
Israel was offered KC-135s again in 2013, after turning down the aging aircraft twice due to expense of keeping them flying. The IAF again rejected the offered KC-135Es, but said that it would consider up to a dozen of the newer KC-135Rs.
Besides its primary role as an inflight aircraft refueler, the KC-135, designated NKC-135, has assisted in several research projects at the NASA Dryden Flight Research Center at Edwards Air Force Base, California. One such project occurred between 1979 and 1980 when special wingtip "winglets", developed by Richard Whitcomb of the Langley Research Center, were tested at Dryden, using an NKC-135A tanker loaned to NASA by the Air Force. Winglets are small, nearly vertical fins installed on an aircraft's wing tips. The results of the research showed that drag was reduced and range could be increased by as much as 7 percent at cruise speeds. Winglets are now being incorporated into most new commercial and military transport jets, as well as business aviation jets.
NASA also has operated several KC-135 aircraft (without the tanker equipment installed) as their famed Vomit Comet zero-gravity simulator aircraft. The longest-serving (1973 to 1995) version was KC-135A, AF Ser. No. 59-1481, named Weightless Wonder IV and registered as N930NA.
The Air Force projected that E and R models have lifetime flying hour limits of 36,000 and 39,000 hours, respectively. According to the Air Force, only a few KC-135s would reach these limits by 2040, when some aircraft would be about 80 years old. The Air Force estimated that their current fleet of KC-135s have between 12,000 to 14,000 flying hours on them-only 33 percent of the lifetime flying hour limit.
Between 1993 and 2003, the amount of KC-135 depot maintenance work doubled, and the overhaul cost per aircraft tripled. In 1996, it cost $8,400 per flight hour for the KC-135, and in 2002 this had grown to $11,000. The Air Force’s 15-year cost estimates project further significant growth through fiscal year 2017. KC-135 fleet operations and support costs are estimated to grow from about $2.2 billion in fiscal year 2003 to $5.1 billion (2003 dollars) in fiscal year 2017, an increase of over 130 percent, which represents an annual growth rate of about 6.2 percent.
In 2006, the KC-135E fleet was flying an annual average of 350 hours per aircraft and the KC-135R fleet was flying an annual average of 710 hours per aircraft. The KC-135 fleet is currently flying double its planned yearly flying hour program to meet airborne refueling requirements, and has resulted in higher than forecast usage and sustainment costs. In March 2009, the Air Force indicated that KC-135s would require additional skin replacement to allow their continued use beyond 2018.
The USAF decided to replace the KC-135 fleet. However, the KC-135 fleet is large and will need to be replaced gradually. Initially the first batch of replacement planes was to be an air tanker version of the Boeing 767, leased from Boeing. In 2003, this was changed to contract where the Air Force would purchase 80 KC-767 aircraft and lease 20 more. In December 2003, the Pentagon froze the contract and in January 2006, the KC-767 contract was canceled. This followed public revelations of corruption in how the contract was awarded, as well as controversy regarding the original leasing rather than outright purchase agreement. Then Secretary of Defense Rumsfeld stated that this move will in no way impair the Air Force's ability to deliver the mission of the KC-767, which will be accomplished by continuing upgrades to the KC-135 and KC-10 Extender fleet.
In January 2007, the U.S. Air Force formally launched the KC-X program with a request for proposal (RFP). KC-X is first phase of three acquisition programs to replace the KC-135 fleet. On 29 February 2008, the US Defense Department announced that it had selected the EADS/Northrop Grumman "KC-30" (to be designated the KC-45A) over the Boeing KC-767. Boeing protested the award on 11 March 2008, citing irregularities in the competition and bid evaluation. On 18 June 2008, the US Government Accountability Office sustained Boeing's protest of the selection of the Northrop Grumman/EADS's tanker. In February 2010, the US Air Force restarted the KC-X competition with the release of a revised request for proposal (RFP). After evaluating bids, the USAF selected Boeing's 767-based tanker design, with the military designation KC-46, as a replacement in February 2011.