A look at what made Jack Kirby famous.
And I don't think either Lee or Kirby ever did come up with it. Wasn't it Ditko, in Tales to Astonish?
Robert is right, it was Steve Ditko came up with that. Ditko had a way of seeing what worked in a character and what didn't. Rather than having some external force cause the change, Ditko made it internal and made the Hulk what he is today. He also did that with Iron Man, giving him a thinner, more versatile, red costume and made him the jet setting hero he is today. And we don;t think of it, but Dr. Strange is probably a remake of Dr. Droom, Marvel's first mystic hero introduced in Amazing Adventures a few months before the first FF. Ditko inked Kirby's Droom. How lucky was Marvel to have two of the best at the same time?