Bullet Bills are common enemies in the Mario series, debuting in Super Mario Bros.. They quickly fly in a straight line, damaging the player on contact, and like most enemies they can be defeated by simply jumping on them. They are most commonly shot by Bill Blasters, though in rare cases they appear on their own. In later games, homing and bigger versions were introduced.
While Bullet Bills are normally enemies in the Mario series, the Bullet Bill as a usable item hails from the Mario Kart series, starting in Mario Kart DS. It is a medium-rare item available to racers near the back of the race that transforms them into a Bullet Bill when used, placing them on autopilot and rendering them invincible with increased speed for several seconds.
Bullet Bill was mentioned in the Banzai Bill trophy.
The Bullet Bill commonly appears as an enemy in The Subspace Emissary. They fly from offscreen, exploding upon impact with a character or on the ground. They also appear in Donkey Kong and Diddy Kong's introduction, in which three of them are destroyed by Diddy Kong. They can be reflected, but will often quickly recover and continue their attack. Some of them are homing, following players until they hit. Unlike in the Mario games, Bullet Bills cannot be defeated by jumping on them.
The Bullet Bill appears on the ground and can be picked up. Upon use, instead of throwing the item itself, the user transforms into a large Bullet Bill and flies in a straight line in whatever direction the control stick/circle pad was pointed in on release. Upon traveling about the length of Final Destination, the user returns to normal, and will recoil backwards to help with returning to stage unless they have aimed directly upwards. The Bullet Bill does 25% damage and delivers high knockback that can KO under 80%, but firing oneself directly downwards or towards a blast zone can cause a self-destruct. If the bullet hits a solid surface, it will produce an explosion that deals 30% damage. It is possible to drive the bullet into the floor with an opponent in between so that the bullet and explosion hit consecutively, dealing a total of 55% damage.
Notably, where the Bullet Bill can be directed is not limited to the cardinal (up, down, left, right) or intermediate (perfect diagonal) directions and can be aimed between such, allowing for more accuracy when using the item. Additionally, the player is considered a projectile while transformed into a Bullet Bill, meaning characters with Reflectors can simply make the Bullet Bill fling back in the direction it came from while damaging the user in that form or attacks with priority will nullify the Bullet Bill and cause it to pass through the attacker, but it can still do damage afterwards.
Humorously, the user is forced into their reeling animation once the Bullet Bill transformation ends, although this is cosmetic and can be cancelled into anything else, such as an attack or jump.
When you try to throw this item, you'll instead transform into a Bullet Bill and fly in the direction you were throwing. Anyone in your path will be knocked away. Be careful, though, since you fly fast and can barely steer—not to mention that foes can deflect you. KO them, not yourself!
Turn into a Bullet Bill and charge into your enemies to send them flying. The direction you fly in is pretty much set when you use it. You'll be unstoppable, but on the other hand, you'll fly so fast that you might end up a victim of your own bullet-y...ness. Anyway, be careful!
Bullet Bills also appear as enemies. They retain their homing behavior from Brawl, though they can no longer be reflected. They deal 3% damage upon impact, or more if they collide with a surface and explode. In addition to being found on their own, they can also be fired from Bill Blasters, in which case they do not home in on the player.
A large number of Bullet Bills can appear as a random event.
Bullet Bills appear as part of the Mario-themed Vs. Enemy Team final battle, where some of them fly forward like as if they were shot out of a Bill Blaster rather than homing in on the player.
The Bullet Bill returns as an item, retaining the same functionality as in Smash 4. It also appears as a spirit.