The very first was the -10 (-11 / -12 etc.). It had early and weak
JT8D(-1?) engines and simple wing without leading edge slats and short fuselage.
Next major version was the -30 with longer fuselage, more powerful engines and much revised wing with leading edge slats.
The -40 was a stretched -30 with even more powerful JT8D-9 or -15 engines.
The -20 was a special short field version combining the -10 fuselage, -30 wing and -9 engines.
The -50 was a further stretch of the -40 with JT8D-15 or -17 engines.
And it all "ended" with the DC-9 Super 80, which came with a much revised wing of greater span, even longer fuselage and JT8D-209 or -217 engines. It was soon renamed MD-80 (MD-81 /-82 etc.) following the merge with McDonnel.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs
You can find more information on the history of the DC-9 if you go to www.boeing.com and click on "Commerical Aircraft". Under that, you should find listings for planes from the DC-8, DC-9, DC-10, to the Boeing 707, 717 all the way up to the 777.
Aviation is proof that, given the will, we have the capacity to achieve the impossible.