Bullet Bills appear as an enemy in an event that deals with defeating as much enemies as players can. (mariowiki.com)
Multiple Bullet Bills can appear during the race if got at different times. (mariowiki.com)
In certain galaxies, Yoshi can use his tongue to capture an incoming Bullet Bill in his mouth for a short time, and then spit it back out at other incoming enemies or bosses. (mariowiki.com)