The C-17 was conceived in an era when Vietnam and the 1973 Yom Kippur airlift had exposed the limits of American air mobility. The C-X competition sought a jet that could carry the Army’s outsize equipment, including tanks, and deliver it straight to forward airfields. McDonnell Douglas’s winning design used externally blown flaps, in which engine exhaust is directed over large slotted flaps to create extra lift at low speed, a technique proven on the YC-15 prototypes of the 1970s.

Budget pressures slowed full-scale development, which did not begin in earnest until the mid-1980s. Problems with the wing, software and weight, combined with cost growth, made the programme a target in Washington in the early 1990s. After the 1993 intervention by the Defense Department, the company reorganised its management and production, and a demanding reliability evaluation in 1995 helped persuade the Air Force to buy a full fleet. What had been one of the most criticised procurement programmes of its day ended as one of the most valued aircraft in the inventory.

The Royal Air Force was the first export customer, initially leasing four aircraft from 2001 for No. 99 Squadron at RAF Brize Norton and later buying them and adding more, for a total of eight. Australia’s No. 36 Squadron at Amberley received its first in 2006 and also built up a fleet of eight, used for missions from Afghanistan to Antarctica and for disaster relief across the Pacific. Canada bought five as the CC-177, and India’s fleet of eleven made it the largest operator outside the United States.

The most unusual operator is the Strategic Airlift Capability, a consortium of NATO and partner nations, including non-aligned Sweden and Finland at the time, that jointly owns three C-17s based at Pápa in Hungary. Flown by the multinational Heavy Airlift Wing since 2009, the aircraft carry national markings of Hungary but fly missions for each member according to its share of flight hours, giving smaller nations access to strategic airlift they could not afford alone.

• C-17A: the only production version, built in blocks with progressive improvements to avionics, defensive systems and structure.
• Extended-range aircraft: later production added a centre-wing fuel tank, retrofitted to earlier aircraft.
• CC-177: Royal Canadian Air Force designation.
• Globemaster III (RAF, RAAF): export aircraft built to the same standard as USAF C-17A.
• BC-17X / MD-17: proposed commercial versions; none entered service.
• C-17B: proposed tactical version with improved short-field capability; not built.

The C-17 is a costly aircraft by any measure. The US Air Force fact sheet gives a unit cost of about US$218 million in fiscal 1998 dollars, and in 2021 Boeing received a ten-year sustainment contract worth US$23.8 billion for the global fleet. Hourly costs, as reflected in the Air Force’s published reimbursement rates, run to tens of thousands of dollars; we give a hedged range of roughly US$20,000 to 25,000 per hour, with fully burdened figures that include depreciation and personnel considerably higher.

The case for the aircraft has always rested on what it replaces. By delivering cargo straight to the point of need, a C-17 can avoid a trans-shipment onto smaller aircraft and the ground handling that goes with it, and its three-person crew is smaller than those of the C-141 and C-5. For smaller air forces, the Strategic Airlift Capability’s shared fleet shows another way to spread the cost.