The Missile Bill (also called the Bull's Eye-Bill or the Heat-Seeking Bullet Bill) is a enemy in the Yoshi series appearing in New Super Mario Bros. Wii and Yoshi's Woolly World it is a red Bullet Bill that originally debuted in Super Mario Bros. 3. Some argue that the color is actually orange, but arguing semantics always proves to be a waste of time. Missile Bills are similar to Biting Bullet Bills, an enemy found in Yoshi's Island: Super Mario Advance 3.
New Super Mario Bros. Wii
Missile Bills are seen in New Super Mario Bros. Wii in the level World 9-3. They are shot out of the Bill Blasters just like a standard Bullet Bill but they home in on Mario whenever they see him. They can be annoying, especially when multiple are fired at once and they start to overwhelm Mario. Mario can easily defeat them by stomping on them or using a Starman. Jumping on a Missile Bill can also boost Mario's jump which he can use to reach higher platforms. Aditionally, freezing them can also create a platform which Mario can use to hop across gaps. The Missile Banzai Bill is also found in the same level.
Yoshi's Woolly World
The Missile Bill appears again in Yoshi's Woolly World exclusively in Knot-Wing the Koopa's Sky Fort they attack by homing in on Yoshi in addition after pursuing Yoshi for some time they flash briefly before they explode they also appear during the boss fight against Knot-Wing the Koopa. They are launched from red Bill Blasters.