Bow aiming animation for entity

I made a very simple custom entity (mob). I gave it Bow in the main hand, in AI section I enabled ranged attacks with default item. I also set entity base as Skeleton.

I just want the mob to behave like a vanilla skeleton, aim the bow and fire it.  Instead it just holds the bow down in it's hand and spits arrows out of it's face. Is there a simple solution to this, or do I have to make custom animations and a custom ranged weapon?