can be done but it does require the proper wood, a circular saw and some heavy duty survey steaks.
A 2x6 or 2x8 would be the best to do this. Then if exterior applications, you need either Redwood or Pressure treated wood, I would suggest the latter.
The longer the board the easier it will bend and be cooperative. I would lay the board on a set of horses and mark off every inch down the board. Then I would set my circular saw to cut the depth of one half way through then with the saw, and cutting straight I would cut on the line all the way every inch all the way down the board.
The cut side or the side with the kerf will be the side that will bend IN, bending out will crack the board. So, take precautions in selecting your side. The board will now bend inward because the kerfs or cuts will allow the board to squeeze in to these cuts. Again, this works better on longer boards like 16 footers. The shorter the board the deeper the cut needs to be, but too deep and the board will crack.
I have built many circle decks doing the same procedures and they will last. The key is to not go too deep and not too close together and using long boards.
Another way, which I have not had good luck with is to thoroughly soak a board in water and set it on a set of horses and slowly adding weight to the center of the board straddling the horses and this will permentaly bend the board, but you do not have allot of control on the bend.