At The Beginning

In the classic Hulk origin story, Bruce Banner helps develop a Gamma Bomb, and while testing it notices the teenager Rick Jones in the blast radius. He manages to save Jones, but is caught in the blast, which turns him into Hulk.

In other origins, Banner instead is injected (or injects himself) with an attempted recreation of the Super Soldier serum, which goes horribly wrong and creates the Hulk.

Some origins mix the two, with a combination of the recreation of the Super Soldier serum and gamma radiation turning Banner into The Hulk.