From the Super Mario Wiki
King Bills are a sub-species of Bullet Bill that first appeared in the final level of New Super Mario Bros. Wii, World 9-8. They are grey in color, with a "goofy" grin as opposed to a menacing grin, and are the largest known Bullet Bills, dwarfing the Banzai Bills.
New Super Mario Bros. Wii
In the game, King Bills are invincible and unable to be beaten with any maneuver or item, and can only be avoided by bouncing above them with clouds or avoiding their paths. Even a Star cannot defeat a King Bill, though it will keep one from harming Mario. The only way a King Bill can be defeated is by jumping on the end-level flagpole, which is proven by the fact that one chases Mario at the end of level 9-8.
The players are warned about the incoming King Bills in the form of groups of Coins shaped like a bullet, and many stone blocks that cannot be destroyed without the King Bill's assistance. King Bills will also destroy most other blocks in the area, such as the Used Blocks, ? Blocks, and Brick Blocks.
Mario must also avoid going too high to avoid the King Bills. Since the whole level has clouds at the bottom that make Mario bounce very high, he occasionally must try to bounce under the stone blocks to avoid going too high and not hit them. At the end of the level, a King Bill will chase Mario, and he must get under it and reach the end-level flagpole. Like Banzai Bills in Super Mario World, King Bill Blasters are not seen on screen.
They are very large in size, taking up nearly the entire screen, which makes it much larger than the previous biggest Bill, the Banzai Bill. It is the biggest non-boss enemy of the game. It is also most likely the second-biggest enemy of the game (including bosses), biggest being Giant Bowser. The King Bill's eye pupils follow Mario's movements.
There is a total of seven King Bills in New Super Mario Bros. Wii, all of them appearing exclusively in the aforementioned level, World 9-8.
New Super Mario Bros. U
King Bills reappear in New Super Mario Bros. U, making their only appearance in the level, Flight of the Para-Beetles. Their flight paths are still marked by coins in the shape of Bullet Bills and their pupils still follow the closest player. This time, however, the King Bills are enveloped in yellow flames similar to that of Mad Banzai Bills, and they move at a slower speed. Only five King Bills make an appearance in the aforementioned level.
New Super Luigi U
King Bills only appear at two levels in New Super Luigi U. They first appear at Larry's Trigger-Happy Castle, chasing after the players as they head to the boss door. Their second appearance is at Para-Beetle Parade. Compared to in New Super Mario Bros. U, the King Bills in New Super Luigi U move at a faster speed.