The 717 was originally the designator for a
tanker version of the 707. Eventually, the Air Force gave Boeing a production contract, and so the aircraft became known as the
KC-135. After the
Boeing 720 (a shorter version of the 707), the company decided to designate all of its transport planes with double-7 numbers, starting with the
727 and later the
747,
767,
757,
737, etc.
The current 717 was originally built by McDonnell-Douglas, and is an evolved version of the DC-9. Originally, Douglas stretched the DC-9's fuselage and called the aircraft the DC-9 Super 80 (American still uses this name). The aircraft's next few modifications were known as the MD-82, MD-83, MD-88, and MD-90 before Boeing bought out the line and renamed the aircraft.
The 717 is long and skinny: it only seats five abreast, compared to the 737 which seats six abreast. Many airlines still operate older 717 models, but most of the new orders coming into Boeing's plant are for the 737.