Bullet Bill is an enemy that appears in Whomp's Fortress after defeating King Whomp. Bullet Bills are fired from a Bullet Bill Blaster at the top of Whomp's Fortress and attack Mario by flying into him. They deal 3 damage and cannot be defeated.
The Bullet Bill Blaster is purely visual, as the Bullet Bill itself handles all its movement and is simply placed in such a spot to give off the illusion that the Blaster is firing it. The Bullet Bill is initialized with a velocity of 0 and positioned at its home. If Mario is within 45 degrees of the Bullet Bill’s line of sight and his distance from the Bullet Bill is between 400 and 1500 units, the Bullet Bill Blasting action will begin. The Bullet Bill moves forward at 3 units/frame for 40 frames, then alternates moving backward 3 units and forward 3 units per frame for another 10 frames. At frame 50, the bullet bill firing sound plays, the Bullet Bill smoke spawns, and the Bullet Bill begins moving forward at 30 units/frame. After 71 frames, the Bullet Bill is able to interact with walls and running into one will make it disappear and reset to its original position. If Mario’s distance from the Bullet Bill is greater than 300 units, the Bullet Bill will rotate its angle toward Mario at 1.4 degrees per frame. After an additional 100 frames, the Bullet Bill disappears into mist and is reset to its initial state and position. If the Bullet Bill strikes Mario, the Bullet Bill becomes intangible and flies backwards at 30 units/frame while moving upward 20 units/frame and rotating. After 90 frames, the Bullet Bill is reset to its initial state and position.