Two crew
McDouglas promised ALPA to never build another two crew plane after the DC-9. When bigger engines became available in about 1978, DAC proceeded to design and build the stretched DC-9, calling it the DC9-80. ALPA howled. By this time, the three crew 767 and 757 were in initial production. Then, lo and behold, accident rates showed the two crew 737 had the best record, followed by the DC-9. The 3-crew 737 was not as good. That ended the controversy.
The UAL and other 3-crew 767s were modified at Boeing to 2-crew, and the DC9-80 got a name change to MD-80. Nobody could imagine 2-crew, 4-engine cargo planes at the time, you can bet.
GB