Starting in 1950 the USAF operated the world's first production aerial tanker, the Boeing KC-97 Stratofreighter, a gasoline fueled piston-engined Boeing Stratocruiser (USAF designation C-97 Stratofreighter) with a Boeing-developed flying boom and extra kerosene (jet fuel) tanks feeding the boom.

[4] In 1954 USAF's Strategic Air Command (SAC) held a competition for a jet-powered aerial refueling tanker.

[5] Since Boeing's proposal was already flying, the KC-135 could be delivered two years earlier and Air Force Secretary Harold E. Talbott ordered 250 KC-135 tankers until Lockheed's design could be manufactured.

The first aircraft flew in August 1956 and the initial production Stratotanker was delivered to Castle Air Force Base, California, in June 1957.

The USAF EC-135 Looking Glass was subsequently replaced in its role by the U.S. Navy E-6 Mercury aircraft, a new build airframe based on the Boeing 707-320B.

This allows the use of more fuel for proper combustion and creates more thrust for short periods of time, similar in concept to "War Emergency Power" in a piston-engined aircraft.

The KC-135E fleet has since either been retrofitted as the R-model configuration or placed into long-term storage ("XJ"), as Congress has prevented the USAF from formally retiring them.

[8] The second modification program retrofitted 500 aircraft with new CFM International CFM56 (military designation: F108) high-bypass turbofan engines produced by General Electric and Safran.

[citation needed] Upgrading the remaining KC-135Es into KC-135Rs is no longer in consideration; this would have cost approximately US$3 billion, $24 million per aircraft.

Among these was the Pacer-CRAG program (compass, radar and GPS) which ran from 1999 to 2002 and modified all the aircraft in the inventory to eliminate the Navigator position from the flight crew.

The program development was done by Rockwell Collins in Iowa[11] and installation was performed by BAE Systems at the Mojave Airport in California.

The only external difference between a KC-135R and a KC-135T is the presence of a clear window on the underside of the empennage of the KC-135T where a remote controlled searchlight is mounted.

This allows the tanker to refuel two receivers at the same time, which increases throughput compared to the boom drogue adapter.

[citation needed] A number of KC-135A and KC-135B aircraft have been modified to EC-135, RC-135 and OC-135 configurations for use in several different roles (although these could also be considered variants of the C-135 Stratolifter family).

The KC-135R has four turbofan engines, mounted under 35-degree swept wings,[17] which power it to takeoffs at gross weights up to 322,500 pounds (146,300 kg).

A special shuttlecock-shaped drogue, attached to and trailing behind the flying boom, may be used to refuel aircraft fitted with probes.

The KC-135 was initially purchased to support SAC bombers, but by the late 1960s, in the Southeast Asia theater, the KC-135 Stratotanker's ability as a force multiplier came to the fore.

KC-135s continued their tactical support role in later conflicts such as Operation Desert Storm and current aerial strategy.

[18] Following a major USAF reorganization that resulted in the inactivation of SAC in 1992, most KC-135s were reassigned to the newly created AMC.

One such project occurred between 1979 and 1980 when special wingtip "winglets", developed by Richard Whitcomb of the Langley Research Center, were tested at Armstrong, using an NKC-135A tanker loaned to NASA by the Air Force.

The results of the research showed that drag was reduced and range could be increased by as much as 7 percent at cruise speeds.

[citation needed] In March 2009, the Air Force indicated that KC-135s would require additional skin replacement to allow their continued use beyond 2018.

This move followed public revelations of corruption in how the contract was awarded, as well as controversy regarding the original leasing rather than outright purchase agreement.

The then Secretary of Defense Rumsfeld stated that that move would in no way impair the Air Force's ability to deliver the mission of the KC-767, which would be accomplished by implementing continuing upgrades to the KC-135 and KC-10 Extender fleet.

[35] On 18 June 2008, the US Government Accountability Office sustained Boeing's protest of the selection of the Northrop Grumman/EADS's tanker.

[36] In February 2010, the US Air Force restarted the KC-X competition with the release of a revised request for proposal (RFP).

[47] The flight engineer's position was removed when the aircraft were modified to KC-135 standards but they retained their electrically powered wing flap secondary (emergency) drive mechanism and second air conditioning pack which had been used to cool the RC-135As on-board photo-mapping systems.

[48] Later re-engined with Pratt & Whitney TF33 engines and a cockpit update to KC-135E standards in 1990 and were retired to the 309th AMARG at Davis-Monthan AFB, AZ in 2007.

KC-135As and some KC-135Es re-engined with CFM56 engines, more than 417 converted[1] Receiver-capable KC-135R Stratotanker; eight modified with either a Boeing or LTV receiver system and a secure voice SATCOM radio.

Data from USAF Fact Sheet,[1] Boeing.com : KC-135[2]General characteristics Performance Related development Aircraft of comparable role, configuration, and era