The Boeing KC-135 Stratotanker is a military aerial refueling aircraft. It and the Boeing 707 airliner were developed from the Boeing 367-80 prototype. It is the predominant variant of the C-135 Stratolifter family of transport aircraft. The KC-135 was the US Air Force's first jet-powered refueling tanker and replaced the KC-97 Stratofreighter. The KC-135 was initially tasked with refueling strategic bombers, but was used extensively in the Vietnam War and later conflicts such as Operation Desert Storm to extend the range and endurance of US tactical fighters and bombers.

The KC-135 made its first flight on August 31, 1956 and entered service with the United States Air Force in 1957; it is one of six military fixed-wing aircraft with over 50 years of continuous service with its original operator. The KC-135 is supplemented by the larger KC-10. Studies have concluded that many of the aircraft could be flown until 2040, although maintenance costs have greatly increased. The aircraft will eventually be replaced by the Boeing KC-46 Pegasus.

The KC-135R has four turbofan engines, mounted under 35-degree swept wings, which power it to takeoffs at gross weights up to 322,500 pounds. Nearly all internal fuel can be pumped through the tanker's flying boom, the KC-135's primary fuel transfer method. A special shuttlecock-shaped drogue, attached to and trailing behind the flying boom, may be used to refuel aircraft fitted with probes. This apparatus is significantly more unforgiving of pilot error in the receiving aircraft than conventional trailing hose arrangements; an aircraft so fitted is also incapable of refueling by the normal flying boom method until the attachment is removed. A boom operator stationed in the rear of the aircraft controls the boom while lying prone. A cargo deck above the refueling system can hold a mixed load of passengers and cargo. Depending on fuel storage configuration, the KC-135 can carry up to 83,000 pounds of cargo.

When it was first delivered to the United States Air Force, the KC-135 fleet was powered by four Pratt & Whitney J57 turbojet engines.  Each engine delivered approximately 12,000 pounds of thrust and a fully loaded KC-135 needed a 12,000 foot runway to safely get off the ground.  Water injection was sometimes needed to gain additional thrust in some situations.  These aircraft were designated KC-135A and served until the 1980s, when two programs were initiated to re-engine the aircraft.  Approximately 138 aircraft were re-engined with Pratt & Whitney JT-3D engines taken from surplus retired 707s to become the KC-135E while about 500 aircraft were re-engined with CFM International turbofan engines and designated the KC-135R.  The new CFM engines reduced the noise footprint, increased fuel efficiency, and increased the amount of fuel the KC-135 can offload to receivers. 

As the aircraft has been in service since the late 1950s, 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 eighty 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. 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. After evaluating bids, the USAF selected Boeing's 767-based tanker design, with the military designation of KC-46, as a replacement in February 2011.

In addition to the United States Air Force, the KC-135 is used by the Air Forces of Chile, France, Singapore, and Turkey.

Photos in the slideshow were taken between March 2016 and April 2018.  Apologies for the large copyrights in each photo - this is necessary in this day and age where photos are widely stolen and passed along as if they are one's own.