From an old interview with George Lucas:
In one of the scripts there is a Wookie planet. It's a jungle planet and there was a whole sequence where the Empire had a little outpost on the Wookie planet and Luke [Skywalker] gets involved with the Wookies and he fights the head Wookie. He wins the fight but he doesn't kill the Wookie and the Wookie says, okay, you are going to be the son of the chief and all that kind of stuff. He rallies the Wookies and the Wookies all attack this imperial base. The imperial base has tanks and all kinds of stuff and the Wookies beat them off, and then Luke and Ben [Kenobi] and Han (Solo] and a bunch of people train the Wookies to fly the fighters, and it is the Wookles that go after the Death Star, not the rebels that were on the planet.