The Origin Story:
A patriot desperate to serve his country during WW2, Steve Rogers agrees to undergo an experimental treatment that will bulk him up a bit. Once buff, Rogers becomes Captain America, initially a promotional tool, but eventually he gets the chance to prove his use to the US army as a super-soldier and battle Nazi soldier the Red Skull.
Essential Elements Introduced:
Cap's strength, endurance, agility, speed, reflexes, durability, and healing ability are at the zenith of natural human potential. He also has a lovely shiny shield.
Saved For The Sequels:
Having sacrificed himself to save America, Cap wakes up 70 years later. There's going to be a lot of adjusting to do for the naive 1940s New Yorker.