Boeing has actively marketed the C-17 to many European nations including Belgium, Germany, France, Italy, Spain and the United Kingdom. Of these, the UK was always seen as the most likely customer given its increasingly expeditionary military strategy and global commitments. The Royal Air Force has established an aim of having interoperability and some weapons and capabilities commonality with the United States Air Force. The UK's 1998 Strategic Defence Review identified a requirement for a strategic airlifter. The Short-Term Strategic Airlift (STSA) competition commenced in September of that year, however tendering was canceled in August 1999 with some bids identified by ministers as too expensive (including the Boeing/BAe C-17 bid) and others unsuitable. The project continued, with the C-17 seen as the favorite. The UK Defence Secretary, Geoff Hoon, announced in May 2000 that the RAF would lease four C-17s at an annual cost of £100 million from Boeing for an initial seven years with an optional two year extension. At this point the RAF would have the option to buy the aircraft or return them to Boeing. The UK committed to upgrading the C-17s in line with the USAF so that in the event of them being returned to Boeing the USAF could adopt them.

The first C-17 was delivered to the RAF at Boeing's Long Beach facility on May 17, 2001 and flown to RAF Brize Norton by a crew from No. 99 Squadron which had previously trained with USAF crews to gain competence on the type. The RAF's fourth C-17 was delivered on August 24, 2001. The RAF aircraft were some of the first to take advantage of the new center wing fuel tank.

The RAF declared itself delighted with the C-17 and reports began to emerge that they wished to retain the aircraft regardless of the A400M's progress. Although the C-17 fleet was to be a fallback for the A400M, the UK announced on July 21, 2004 that they have elected to buy their four C-17s at the end of the lease, even though the A400M is moving towards production. They will also be placing a follow-on order for one aircraft, though there may be additional purchases later.While the A400M is described as a "strategic" airlifter, the C-17 gives the RAF true strategic capabilities that it would not wish to lose, for example a maximum payload of 77,000 kg compared to the Airbus' 37,000 kg.

The Ministry of Defence (MoD) announced on August 4, 2006 that they had ordered an additional C-17 and that the four aircraft on lease will be purchased at the end of the current contract in 2008. The fifth aircraft was delivered on February 22, 2008. Due to fears that the A400M may suffer further delays, the MoD is planning to acquire three more C-17s (for a total of eight) for delivery in 2009-2010, provided that the U.S. Air Force places a follow-on order extending through the same time periodOn July 26, 2007, Defence Secretary Des Browne announced that the MoD intends to order a sixth C-17 to boost operations in Iraq and Afghanistan.] On December 3, 2007, the MoD announced a contract with Boeing for a sixth C-17 to be delivered in mid-2008.

In RAF service the C-17 has not been given an official designation (e.g. C-130J referred to as Hercules C4 or C5) due to its leased status, but is referred to simply as the C-17. Following the end of the lease period the four aircraft will assume an RAF designation, most likely "Globemaster C1".

General characteristics
Crew: 3: 2 pilots, 1 loadmaster

102 troops or 36 litter and 54 ambulatory patients
Payload: 170,900 lb (77,519 kg) of cargo distributed at max over 18 463L master pallets or a mix of palletized cargo and vehicles
Length: 174 ft (53 m)
Wingspan: 169.8 ft (51.75 m)
Height: 55.1 ft (16.8 m)
Wing area: 3,800 ft² (353 m²)
Empty weight: 282,500 lb (128,100 kg)
Max takeoff weight: 585,000 lb (265,350 kg)
Powerplant: 4× Pratt & Whitney F117-PW-100 turbofans, 40,440 lbf (180 kN) each
Fuel capacity: 35,546 US gal (134,556 L)

Cruise speed: Mach 0.76 (450 knots, 515 mph, 830 km/h)
Range: 2,420 nmi[64] (2,785 mi, 4,482 km)
Service ceiling 45,000 ft (13,716 m)
Max wing loading: 150 lb/ft² (750 kg/m²)
Minimum thrust/weight: 0.277