In 1972, the U.S. Air Force issued the Advanced transport STOL
transport program for a new medium airlifter that capable of operations
into battle zones with short, temporary airfields. Two proposals were
accepted for construction as the Boeing YC-14 and McDonnell Douglas
YC-15 prototypes. The Boeing design for STOL performance was based on
the use of a supercritical wing, developed by NASA that the high-speed
airflow from the engines tended to cling to the upper surface of the
wing/flap system and was thus directed downward to provide powered lift.
It was the most efficient powered-lift system ever developed. The first
flight of the YC-14 demonstrated superior STOL performance and low-speed
maneuverability. However, by the time the YC-14 was ready for
production, military priorities had changed. The Air Force decided it
needed a larger transport that would fly to standard, conventional
airfields rather than into battle zones. In 1979, the Air Force formally
cancelled the AMST program for both the YC-14 and the YC-15. As a
result, the C-X program, the C-17 evolved.