Airlines operating the A300 on short haul routes were forced to reduce frequencies to try and fill the aircraft. As a result, they lost passengers to airlines operating more frequent narrow body flights. Eventually, Airbus had to build its own narrowbody aircraft (the A320) to compete with the Boeing 737 and McDonnell Douglas DC-9/MD-80. The savior of the A300 was the advent of ETOPS, a revised FAA rule which allows twin-engine jets to fly long-distance routes that were previously off-limits to them. This enabled Airbus to develop the aircraft as a medium/long range airliner.