The 717 was indeed part of the DC-9 family, the program was launched under the MD-95 moniker and it is listed as a variant on the DC-9 type certificate data sheet. Conceived as a true replacement for the smaller OG DC-9s, it was originally intended to be stretched to the MD-80’s size and launch a new generation of DC-9-based airliners with a new set of engines and avionics.
Time ran out for McDonnell Douglas though and the 717 barely avoided being scrapped by Boeing at least once. The 100-seat market was congested, and this caused the folding of other companies entering that segment and limited production runs on others like the 737-600.
Interestingly enough, they are still well-liked by Hawaiian, Qantas, and a few other dedicated operators that continue to fly them. I’ve been lucky enough to catch an inter-island flight on one with Hawaiian. The new B-52J will also use a variant of the Rolls Royce BR700 family of engines that power the 717 and a few large-cabin business jet types.
The DC-9’s heritage can still be seen in the COMAC ARJ21 regional jet, which uses a similar fuselage, nose section, and tailcone. Since it sports a new wing, engines, and avionics, China claims it is an indigenous design, but that claim appears rather dubious since, well, this is China we’re talking about.
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