I am vaguely aware that the US space program wasn't just kicked off after Sputnik. There is a history of balloons and sounding rockets that never seem to get mentioned in the books I've read, and programs like Man In Space Soonest and Project Adam, not to mention pre-WWII rocket clubs around the world, and visionaries like Valier and Ganswindt who rarely seem to get mentioned. I'm just trying to fill in some of the gaps and connect some of the dots.
I'm thinking in particular of the Manhigh project, which lifted men in enclosed gondolas to study human survivability at high altitudes. This article explains that "But without the pioneers of the U.S. Air Force’s Balloon program, NASA may never have achieved all that all that it has to date", without going into specifics. Did it affect the Mercury capsule design? Something else?