The DC-9 has been one of the work-horses of the airline industry since 1965, and has been stretched and re-engined many times. Preliminary design was started in 1962, with first flight in February 1965. The airplane entered service with Delta Airlines later that year. The last delivery of one of its derivatives, the MD-87, was in December 1999.
The design is used in many of the examples in Schaufele's "The Elements of Aircraft Preliminary Design". This textbook is used in undergraduate senior year or graduate level courses at several universities, including California State University Long Beach, University of California Irvine, and Arizona State University.