The MD-80 is a popular series and an improved development of the McDonnell Douglas DC-9. In 1975, the origin of the MD-80 can be traced back to the series of testing of the DC-9, fitted with improved more efficient, higher bypass ratio JT8D-200 series turbofans. The MD-80 designation is a generic designation for the series and doesn't apply to a certain model type. Derived from the DC-9, the McDonnell Douglas MD-80 is a twin-engine, medium-range, single-aisle commercial jet airplane. Introduced commercially in October 1980 by Swissair, it was designed for frequent, short-haul flights for 130 to 172 passengers, depending on plane version and seating arrangement. It first flew on October 19, 1979, and production ended in 1999. The MD80 has been used by airlines around the world, one of which was Northwest Airlines. Northwest is the world's fourth largest airline in terms of scheduled passenger miles flown. It acquired the MD-80 in 1986 and retired the aircraft in 1999.