The Mad Bomber is a single unit who belongs to no player nor army. He starts the game on the center of the map. If he receives any damage during a battle, he immediately explodes in the same phase, causing a point of damage to units in all 6 neighboring hexes (except headquarters). He then will be drawn as the first tile by the next player in turn order after the one who caused the bomber to be hit, and that player can deploy him on the map again. If more than one player hits him in the same initiative phase of the battle, then he is knocked out of the game.
For more information, visit the BGG listing.