The Boeing KC-46 Pegasus is an American military aerial refueling and strategic military transport aircraft developed by Boeing from its 767 jet airliner. In February 2011, the tanker was selected by the United States Air Force (USAF) as the winner in the KC-X tanker competition to replace older Boeing KC-135 Stratotankers. The first aircraft was delivered to the USAF in January 2019.

The USAF intends to procure 179 tankers by 2027. The Air Force indicated that the number of KC-46A aircraft to be procured had increased to 188 which is the absolute maximum number available under the original deal. The Air Force has also elected to pursue a "Tanker Production Extension Program" which will lead to a new contract with Boeing for up to 75 new KC-46A. The total airfare program would grow to 288 KC-46A if all options are exercised.

In 2001, the U.S. Air Force began a procurement program to replace around 100 of its oldest KC-135E Stratotankers, and selected Boeing's KC-767. The Boeing tanker received the KC-767A designation from the United States Department of Defense in 2002 and appeared in the 2004 edition of DoD model designation report. The USAF decided to lease 100 KC-767 tankers from Boeing.

U.S. Senator John McCain and others criticized the draft leasing agreement as being wasteful and problematic. In response to protests, the USAF struck a compromise in November 2003, whereby it would purchase 80 KC-767s and lease 20 more. In December 2003, the Pentagon announced a freeze on the program over an investigation into alleged corruption that led to the jailing of one of its former procurement executives who applied to work for Boeing. The KC-767A contract was canceled by the DoD in January 2006.

In 2006, the USAF released a request for proposal (RFP) for a new tanker program, KC-X, to be selected by 2007. Boeing announced it may enter a higher capability tanker based on the Boeing 777, named the KC-777 Strategic Tanker. Airbus partnered with Northrop Grumman to offer the Airbus A330 MRTT, the tanker version of the A330, which was marketed to the USAF under the designation KC-30. In January 2007, the USAF issued the KC-X Aerial Refueling Aircraft RFP, calling for 179 tankers, four system development and demonstration and 175 production, in a contract worth an estimated US$40 billion (~$58.1 billion in 2024). Northrop and EADS expressed dissatisfaction at how the RFP was structured and threatened to withdraw, leaving only Boeing in the running.

In February 2007, Boeing announced it was offering the KC-767 Advanced Tanker for the KC-X, stating that the KC-767 was a better fit than the KC-777 for the requirements. In April 2007, Boeing submitted its KC-767 tanker proposal to USAF. The KC-767 offered for this KC-X round was based on the in-development 767-200LRF (Long Range Freighter), rather than the -200ER on which Italian and Japanese KC-767 aircraft are based, differing by combining the -200ER fuselage, -300F wing, gear, cargo door and floor, -400ER digital flightdeck and flaps, uprated engines, and "sixth-generation" fly-by-wire fuel delivery boom.

Boeing submitted its final proposal in January 2008. In February 2008, the DoD chose the KC-30 over the KC-767, the USAF subsequently designated it KC-45A. Boeing submitted a protest to the United States Government Accountability Office (GAO) in March 2008 and waged a public relations campaign in support of their protest. In June, after USAF admissions on bidding process flaws, the GAO upheld Boeing's protest and recommended the contract be rebid. In July 2008, Defense Secretary Robert Gates announced that the USAF would reopen bidding, and put the contract into an "expedited recompetition" with Defense Undersecretary John Young in charge of the selection process, not the USAF. A draft of the revised RFP was provided to contractors in August 2008 for comments. However, in September 2008, the DoD canceled the KC-X solicitation.

In September 2009, the USAF began a new round of bids with a clearer set of criteria, including reducing the number of requirements from 800 to 373 in an attempt to simplify the process and allow a more objective decision to be made. In March 2010, Boeing announced it would bid the KC-767 for the new KC-X round. EADS stated in April 2010 it would submit a bid without Northrop Grumman as a U.S. partner. Boeing submitted its KC-767 "NewGen Tanker" bid, based on the 767-200 with an improved version of the KC-10's refueling boom, and cockpit displays from the 787, in July 2010. Boeing submitted a revised bid in February 2011.