According to a 1UP feature celebrating the 25th anniversary of the Nintendo Entertainment System. To quote Mario‘s creator:

“During much of development, the controls were A for shoot bullets, B to dash, and up on the control pad to jump,” said Miyamoto. “The bullets wound up becoming fireballs later — 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. In the end, we realized that being able to shoot all the fireballs you want while running gave Mario too much of an advantage, so instead we had it so you shoot only one fireball when you start running. That freed up the A button, and we made that the jump button. I really wanted to have A be the action button and make you press up to jump, but it definitely worked out better for Mario in the end.”

Read the rest [here]

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