The McDonnell Douglas MD-90 is an American five-abreast single-aisle airliner developed by McDonnell Douglas from its successful model MD-80. The airliner was produced by the developer company until 1997 and then by Boeing Commercial Airplanes. It was a stretched derivative of the MD-80 and thus part of the DC-9 family. After the more fuel-efficient IAE V2500 high-bypass turbofan was selected, Delta Air Lines became the launch customer on November 14, 1989. The MD-90 first flew on February 22, 1993, and the first delivery was in February 1995 to Delta.

A narrow-body aircraft or single-aisle aircraft is an airliner arranged along a single aisle, permitting up to 6-abreast seating in a cabin less than 4 metres (13 ft) in width. In contrast, a wide-body aircraft is a larger airliner usually configured with multiple aisles and a fuselage diameter of more than 5 metres (16 ft), allowing at least seven-abreast seating and often more travel classes.