According to Wikipedia, there were some KC135's fitted with Flight Engineer positions.
KC-135D
All four RC-135As (Pacer Swan) were modified to partial KC-135A configuration in 1979.[42][43] The four aircraft (serial numbers 63-8058, 63-8059, 63-8060 and 63-8061) were given a unique designation KC-135D as they differed from the KC-135A in that they were built with a flight engineer's position on the flight deck.[44] The flight engineer's position was removed when the aircraft were modified to KC-135 standards but they retained their electrically powered wing flap secondary (emergency) drive mechanism and second air conditioning pack which had been used to cool the RC-135As on-board photo-mapping systems.[45] Later