A Famitsu interview with Shigeru Miyamoto reveals that using Mario's original controls, pressing Up would jump, with B to dash and A to fire bullets, before the team settled on fireballs. The game would have turned out very differently with these original controls, as the plan was to put shooting stages into the plumber's adventure.
We originally thought about having a shoot-'em-up stage where Mario jumps on a cloud and shoots at enemies, but we dropped it because we wanted to focus on jumping action. The sky-based bonus stages are the remnants of that idea, you could say.
Mario's been in plenty of different genres over the years, from racing to party, but to think his debut could have been part-platformer, part-shooter is surprising to say the least.
Oh, and the Minus World? It's a feature, apparently:
That's a bug, yes, but it's not like it crashes the game, so it's really kind of a feature, too!
Every cloud has a silver lining.