Your hickory board will not straighten out simply by leaving it in the rain and sun. You could clamp it in a reflex form for a month and it still won't change. in order for anything to happen, you must heat the wood. I would use a heat gun to bring it to a temperature where it almost burns your hand but doesn't scorch the wood. bend the wood a little past straight because it will spring back a bit. Leave it in that position until it cools. Before you do any of this, rough out the bow and get it floor tillered, it will be much easier to heat correct. Also, Hickory must be very dry in order to get the maximum from it. Be sure to keep it in a warm, dry place for a few weeks before doing work on it. My first bow was from a hickory board 4 years ago, still shoot it often. Hope this helps.
Cody