Riddle #7

There are 3 black hats and 2 white hats in a box. Three men (we will call them Tom, Bob, and Bill) each reach into the box and place one of the hats on his own head. They cannot see what color hat they have chosen. The men are situated in a way that Tom can see the hats on Bob and Bill's heads, Bob can only see the hat on Bill's head and Bill cannot see any hats. When Tom is asked if he knows the color of the hat he is wearing, he says no. When Bob is asked if he knows the color of the hat he is wearing he says no. When Bill is asked if he knows the color of the hat he is wearing he says yes and he is correct. What color hat and how can this be? There is no play on words and there are no tricks.