Built in response to a U.S. Air Force request for an Advanced Medium STOL (Short Takeoff and Landing) Transport (AMST), McDonnell Douglas built two YC-15 prototypes to replace the Lockheed C-130 Hercules tactical transport. The YC-15 used large double-slotted flaps that extended over 75 percent of the wingspan to enhance STOL capabilities. To save costs, it used a modified DC-8 nosewheel unit and the DC-10 cockpit, adapted for a two-person crew, with two lower windows for visibility during short-field landings. The YC-15 introduced a number of innovative features, such as externally blown flaps, which used double-slotted flaps to direct part of the jet exhaust downwards, while the rest of the exhaust passed through and downward over the flaps, introducing the Coanda effect. However, the Air Force decided against the AMST aircraft and terminated the program. This resulted in the development of the C-X (Cargo Transport Aircraft-Experimental), the future C-17.