The story starts with John “Jack” L. Smith. At Studebaker, he ran the Mobilgas Economy Runs; he joined Chrysler in 1957 and rose through product planning, retiring in 1980 as chief engineer of vehicle emissions and fuel economy planning (in 1972, just a few years after the Road Runner, he evaluated (with humor and accuracy) Wankel engines).
The idea for the Pontiac GTO came from either Jim Wangers, an ad man for the Pontiac LeMans, or John DeLorean; either way, they saw younger buyers modifying their cars and thought of making a performance car from the mid-size class, rather than from the usual full-size range (e.g. Chrysler 300, Dodge D-500, Plymouth Fury). That led to the 1964 Pontiac GTO—and other affording performance models, such as the Cutlass 442.
In 1965, Dodge and Plymouth created dedicated planning groups for their midsize cars, putting Jack Smith in charge of the Plymouth group—and charging him with making a GTO competitor. That became the Plymouth GTX, the highest-trim Belvedere with their biggest engine, a strong but expensive car. Plymouth was seen as too conservative for younger buyers to take the bait; only 12,000 GTX cars sold in 1967, versus 81,000 GTOs (not to mention the Chevy SS and Olds 442).
Chrysler-Plymouth sales chief Bob Anderson reportedly asked writer Brock Yates how to get kids’ attention; Yates told him to strip out every nonessential item from an existing car and put in the biggest engine they had. Nobody had done this kind of bare-bones performance car before, though some at Chrysler had thought about it. Smith’s own car was a low-end Belvedere II with a 383 four-barrel engine, four-on-the-floor transmission, police brakes, and heavy duty suspension. The idea was sound, so they adopted some objective standards for a barebones performance car—all applying the car “as sold,” or, in other words, “bone stock.”