Bombshell Bills first debuted in Paper Mario as an enemy. They are fired from normal Bill Blasters, but while they have the same sprite as normal Bullet Bills they have a yellow colorized effect. They only appear in Bowser's Castle.
Paper Mario: The Thousand-Year Door
Bombshell Bills returned as an enemy that is only fought in the Shadow Palace. In the overworld, they are fired from Bombshell Bill Blasters that often guard doors in hallways of the final dungeon; in order to get past them Mario must use his Paper ability.
Super Paper Mario
Bombshell Bills and Bombshell Bill Blasters were set to be in this game, but they ended up being unused.