Flight Engineers were first put on US airliners due to a CAB ruling requiring them on airliners weighing more than 80,000 pounds and earlier on any US airliners in international service. Some lines, Delta, put on pilots; others like American put on ex-mechanics, United used a mix. All this led to a FEIA strike, the splitting of ALPA, as the American pilots formed the Allied Pilots Association. ALPA broke the FEIA union. At Eastern, we had about 100 former engineers who trained as pilots to keep their jobs. ALPA was insistent that only pilots could be in the cockpit. The PATCO and Continental strike of 1982 were just a warm up by comparison.
Then, there was the three-man commission that decided the B757/767 could be flown by two pilots in the US. One, ALPA; one FAA; one industry.
Interesting history, pick up a copy of Flying the Line by George Hopkins. Out of print, but try Abebooks.com
GF