Peter Sellers was supposed to play Major Kong along with his other roles. There are varying stories on how he was replaced. Either Sellers was not happy with his attempt at portraying a Texas cowboy and relented, or he was injured during the production and was not able to perform the scene where Kong releases the bomb from the plane.
In any case, Kubrick asked his casting director to find him a real cowboy, and Slim Pickens showed up. Some of the cast members thought Pickens was in some sort of cowboy costume, with boots, cowboy hat, and the whole get up. But it was just the way he was!
I can’t imagine an Englishman like Peter Sellers playing a Texas cowboy. Slim Pickens, from Kingsburg, in California’s San Joaquin Valley and the son of a Texan, was a perfect fit.