On the civilian side, there was a big brew-ha about FEs and right from the beginning. Airlines didn’t want them and neither did Douglas because they gave them a competitive advantage over their primary rival, which at the time wasn’t Boeing, but Lockheed.
It a pattern that seems destined to repeat itself through eternity, there were some bad accidents, and with some pushing from ALPA, the government required FEs on piston aircraft over a certain weight, and eventually all turbine powered equipment. AMR was rip • pissed over this, and they never forgave ALPA, which caused problems later on.
How this new rule was implemented was highly variable, because, surprise surprise, the training wasn’t that well defined. Some airlines used pilots, some used mechanics, Eventually this would lead to labor problems because on those lines that used PFEs, they formed their own union (FEIA).
The “crew compliment” issue was one of those big labor deals in the 60s, and led to some strange issues, like requiring a 4 man crew on a 737.
It took a lot of effort to hit the reset button, and by the 70s, almost all major airlines used pilots as FEs, with some PFEs grandfathered in. Smaller outfits and cargo operators continued to use PFEs. AAs split with ALPA in the 60’s was caused primarily by the crew complement issue.