The 737 and DC-9 were certified for two pilot operation. The airline pilots union, ALPA objected to the two pilot operation of these twin engine jets and singled out the 737, they ruled that any US airline operating the 737 had to be flown by 3 pilots.
Very few US airlines ordered the 737 because the DC-9 could be flown by 2 pilots, but the 737 had to be flown by 3 pilots and because of this union rule the DC-9 outsold the 737.
United and Western for many years had flight engineers on their 737's. These planes had no flight engineer station and the FE
sat on the observers seat and reached the same switches the pilots could reach. After the FAA ruled that the 757/767 could be operated safely by 2 pilots, the union dropped the requirement for 3 pilots on the 737