The McDonnell Douglas DC-9 is an American five-abreast, single-aisle aircraft designed by the Douglas Aircraft Company. It was initially produced as the Douglas DC-9 prior to August 1967, after which point the company had merged with McDonnell Aircraft to become McDonnell Douglas.

Following the introduction of its first jetliner, the high-capacity Douglas DC-8, in 1959, Douglas was interested in producing an aircraft suited to smaller routes. As early as 1958, design studies were conducted; approval for the DC-9, a smaller all-new jetliner, came on April 8, 1963. The DC-9-10 first flew on February 25, 1965, and gained its type certificate on November 23, to enter service with Delta Air Lines on December 8.

The DC-9 is powered by two rear-mounted Pratt & Whitney JT8D low-bypass turbofan engines under a T-tail for a cleaner wing aerodynamic. It has a two-person flight deck and built-in airstairs to better suit smaller airports. The aircraft was capable of taking off from 5,000 ft runways, connecting small cities and towns in the jet stream of air travel where jet service was previously impossible. The Series 10 aircraft are 104 ft (32 m) long for typically 90 coach seats. The Series 30, stretched by 15 ft (4.5 m) to seat 115 in economy, has a larger wing and more powerful engines for a higher maximum takeoff weight (MTOW); it first flew in August 1966 and entered service in February 1967. The Series 20 has the Series 10 fuselage, more powerful engines, and the Series 30's improved wings; it first flew in September 1968 and entered service in January 1969. The Series 40 was further lengthened by 6 ft (2 m) for 125 passengers, and the final DC-9-50 series first flew in 1974, stretched again by 8 ft (2.5 m) for 135 passengers. When deliveries ended in October 1982, 976 had been built. Smaller variants competed with the BAC One-Eleven, Fokker F28, and Sud Aviation Caravelle, and larger ones with the original Boeing 737.

The original DC-9 was followed by the second generation in 1980, the MD-80 series, a lengthened DC-9-50 with a larger wing and a higher MTOW. This was further developed into the third generation, the MD-90, in the early 1990s, as the fuselage was stretched again, fitted with V2500 high-bypass turbofans, and an updated flight deck. The shorter and final version, the MD-95, was renamed the Boeing 717 after McDonnell Douglas's merger with Boeing in 1997; it is powered by Rolls-Royce BR715 engines. The DC-9 family was produced between 1965 and 2006 with a total delivery of 2441 units: 976 DC-9s, 1191 MD-80s, 116 MD-90s, and 155 Boeing 717s. As of August 2022, 250 aircraft remain in service: 31 DC-9s (freighter), 116 MD-80s (mainly freighter), and 103 Boeing 717s (passenger), while the MD-90 was retired without freighter conversion.

During the late 1950s, Douglas Aircraft studied a short- to medium-range airliner to complement their then-sole jetliner, the high-capacity, long-range DC-8 (DC stands for Douglas Commercial). The Model 2067, a four-engined aircraft sized for medium-range routes was studied in depth, but work on it was abandoned after the proposal did not receive enough interest from airlines. In 1960, Douglas signed a two-year contract with the French aeronautics company Sud Aviation for technical cooperation; under the terms of this contract, Douglas would market and support the Sud Aviation Caravelle and produce a licensed version if sufficient orders were forthcoming from airlines. However, none were ever ordered from the company, leading to Douglas returning to its design studies after the co-operation deal expired.

In 1962, design studies were underway into what would become the DC-9, known as Model 2086. The first envisioned version seated 63 passengers and had a gross weight of 69,000 lb (31,300 kg). This design was changed into what would be the initial DC-9 variant. During February 1963, detailed design work commenced. On April 8, 1963, Douglas announced that it would proceed with the DC-9. Shortly thereafter, Delta Air Lines placed the initial order for the DC-9, ordering 15 aircraft along with options for another 15. By January 1965, Douglas had garnered orders for 58 DC-9 as well as options for a further 44.

Unlike the competing but larger Boeing 727 trijet, which used as many 707 components as possible, the DC-9 was developed as an all-new design. Throughout its development, Douglas had placed considerable emphasis on making the airliner as economic as possible, as well as to facilitate its future growth. The adoption of the Pratt & Whitney JT8D low-bypass turbofan engine, which had already been developed for the Boeing 727, enabled Douglas to benefit from the preexisting investment. Pratt & Whitney had long collaborated with Douglas on various projects, thus their engine was a natural choice for the company. In order to reduce the considerable financial burden of its development, Douglas implemented one of the first shared-risk production arrangements for the DC-9, arranging for de Havilland Canada to produce the wing at its own financial cost in return for promises on prospective future production orders.

The pace of development on the program was rapid. The first DC-9, a production model, flew on February 25, 1965. The second DC-9 flew a few weeks later, with a test fleet of five aircraft flying by July. Several key refinements to the aircraft were made during flight testing, such as the replacement of the original leading-edge slat design to achieve lower drag. The flight test program proceeded at a rapid pace; the initial Series 10 received airworthiness certification from the Federal Aviation Administration on November 23, 1965, permitting it to enter service with Delta Air Lines on December 8.