OMG, Moon. I missed this one during comic book week. {the importance of using tags} And I'm old, but not old enough to have been around when Supes debuted. Heck, my parents weren't even adults (although Dad was close)
Still, I'm pretty up on comics and history. I believe the the bullets bouncing off of him and super strength were what he started out with. You're right that he couldn't fly, but that was added to his legend when the Fleischers animated him. He was given the power of flight to save money on animating him leaping from point to point. He'd have to make over a half million leaps just to make a cross country journey. Not easy to animate for an 8 minute cartoon. I notice some of the modifications from the original have returned lately, including not needing a space suit in outer space, or an air tank underwater.
You're right about Batman being the most popular comic book character. Wizard magazine's readers put him in the #1 spot, with Spider-Man in #2 for the twentieth century. Of course, that list has changed since the new millennium, and some lists (since Wizard died) now have Superman or, on one list, Wolverine, in that spot.