Bullet Bills are living projectiles that have been encountered in many installments of the Super Mario franchise. True to their name, they appear to be large black bullets with a pair of angry eyes and even a pair of arms. They are fired from cannons and will rocket towards Mario at high speed, though there are also variants that can lock onto Mario and home in on him.
In earlier Mario games, Bullet Bills would fly straight forward passing through any and all scenery, and Mario was capable of jumping on them to defeat them. More recent titles have shown them to actually make contact with objects and explode on impact, meaning Mario could no longer touch them without being blown up in the process.