The McDonnell Douglas DC-9 is a twin-engine, short- to medium-range jet airliner that first flew in 1965. Designed for efficiency and reliability, it features a T-tail and rear-mounted engines, which help reduce cabin noise. The aircraft can accommodate around 80 to 150 passengers, depending on the configuration. The DC-9 was widely used by various airlines, including Delta Air Lines and Northwest Airlines, and became popular for its versatility. Over 900 units were produced, and it served as the basis for several derivatives, including the MD-80 series. The last DC-9 was retired from commercial service in 2014.