When the Mushroom Kingdom was overrun by Bowser in Super Mario Bros., Bullet Bills were Bowser's prime ammunition. Bullet Bills first appeared in World 5-1. They were usually shot from a large cannon, the Bill Blaster. Sometimes, Bullet Bills would be shot from random locations somewhere off-screen. The Bill Blasters were placed very strategically in almost all levels, being wherever Mario was. One of the most frustrating places was on the ground, so even a normal-sized Mario would have to jump over them. To defeat a Bullet Bill, the player had to either jump on them, kick a Koopa Shell at them or use a Starman. They were immune to Fire Flowers.