C.W.
This is normal for this age. I think they do it for a reaction and because they can't communicate their frustration. Supposidely paying attention to it makes it worse, but I always catch my son's hand and say "no hit." very firmly and then redirect him to something else. The other thing I would mention, is watch how you and others are playing with him and what he is watching on TV to see if he is unintentionally picking something up there. I say that because my 15 month old was hitting people in the face for a few weeks and then I watched my husband play his "pow pow" game where my husband takes my son's hands and play hits my husband in the face, saying "pow pow" and falling down. It gets a good laugh, but unfortunately my son doesn't realize the difference between play and real and that others may not find this "game" funny. I had my husband stop the game and it has gotten better.
Good Luck to you.