Since producing its first school bus in 1936, virtually all Thomas school bus bodies had been produced in the "conventional" style: a body mated to a cowled truck chassis. While the design was the most popular configuration, the transit-style configuration allowed for a higher passenger capacity (up to 90 passengers). In the early 1970s, Thomas developed the Saf-T-Liner ER (Engine Rear) as an alternative to the Blue Bird All American. In line with other body manufacturers (Carpenter, Superior, Ward, and Wayne), Thomas was dependent on a second-party manufacturer for rear-engine chassis (using Ford, Dodge, GMC, International Harvester and Volvo).