Jerry Siegel and Joe Shuster's original Superman had more in common with Lex Luthor, a villainous slaphead with psychic powers who was bent on world domination. They hit true paydirt when they took the name but completely re-did the character, instead coming up with the world's first superhero.
With his seemingly endless set of superpowers, the alien Kal-El fights against injustice, protects humanity, and spends half of his time pretending to be a normal guy for some reason. Superman is the wellspring from which all later superheroes came from, the patient zero, the Big Bang for comic books.
Totally a rip-off of: Gladiator
Except, like the Big Bang, there had to have been something before it. It's difficult to conceive of - perhaps even impossible for us to get our tiny little human minds around - but the universe didn't just appear out of nothing. And neither did Clark Kent, who was inspired by Greek myths, pulp heroes and Shuster and Simon's own left-leaning political beliefs during the thirties.
Perhaps the strongest influence came from Philip Wylie's 1930 satirical novel Gladiator, wherein a young man is dosed with a super serum by his scientist that makes him as fast as a locomotive, be able to leap forty feet (or, tall buildings) in a single bound, gives his super strength, and makes his skin impenetrable. Those are all actual turns of phrase that originally appeared in Wylie's book. Not only that, but the titular Gladiator kept his secret identity under wraps, living amongst the puny humans as Hugo Danner, who was imbued with a strong moral compass during his early years by his parents who taught him to use his powers for good.
Like Superman in his early years, Danner decided to fight injustice, political corruption and stood up for the little guy whilst running around in his underwear. Gladiator is widely regarded as an influence on Shuster and Siegel - and the identical similes are hard to argue with - but there's never been any confirmation one way or the other.
There are a lot of differences, especially in the character's origins and eventual stories (where Superman becomes Earth's greatest champion, Gladiator becomes frustrated that he isn't living up to his full potential and ends up feeling like a complete failure), but reading Wylie's book and comparing it to the Action Comics star - and all the superheroes he would inspire down the line - it's hard not to see Hugo Danner, rather than Clark Kent, as the real starting point for the modern superhero. That Man of Steel is but a poor imitation.