The McDonnell Douglas YC-15 is a prototype four-engine short take-off and landing (STOL) tactical transport. It was McDonnell Douglas' entrant into the United States Air Force's Advanced Medium STOL Transport (AMST) competition to replace the Lockheed C-130 Hercules as the USAF's standard STOL tactical transport. In the end, neither the YC-15 nor the Boeing YC-14 was ordered into production, although the YC-15's basic design would be used to form the successful McDonnell Douglas (later Boeing) C-17 Globemaster III. Design and development In 1968, the USAF started work on a series of prototype proposals, which would lead to both the AMST project and the Light Weight Fighter. The official Request for proposal (RFP) was issued in January 1972, asking for operations into a semi-prepared field with a payload and a mission radius. For comparison, the C-130 of that era required about for this load. Proposals were submitted by Bell, Boeing, Fairchild, McDonnell Douglas and the Lockheed/North American Rockwell team at this stage of the competition. On 10 November 1972, the two top bids (from Boeing and McDonnell Douglas) were selected. The companies were awarded development contracts for two prototypes each. McDonnell Douglas' prototype was designated YC-15. McDonnell Douglas's design incorporated a supercritical wing, the result of NASA research carried out by the already famous Richard Whitcomb. This wing design dramatically lowers transonic wave drag by as much as 30% compared to more conventional profiles, while at the same time offering excellent low-speed lift. Most contemporary aircraft used swept wings to lower wave drag, but this led to poor low-speed handling, which made them unsuitable for STOL operations. The design team also chose to use externally blown flaps to increase lift. This system uses double-slotted flaps to direct part of the jet exhaust downwards, while the rest of the exhaust passed through the flap and then followed the downward curve due to the Coandă effect. Although the effects had been studied for some time at NASA, along with similar concepts, until the introduction of the turbofan the hot and concentrated exhaust of existing engines made the system difficult to use. By the time of the AMST project, engines had changed dramatically and now provided larger volumes of less-concentrated and much cooler air. The YC-15 prototype was the first jet powered aircraft to use externally blown flaps. For later prototypes, there were several modifications including a computer on the YC-15 was devised that