Sellers was going to play the role of USAF Major Kong.
Sellers ended up playing three of the four roles written for him. He had been expected to play Air Force Major T. J. "King" Kong, the B-52 aircraft commander, but from the beginning Sellers was reluctant. He felt his workload was too heavy and he worried he would not properly portray the character's Texan English accent. Kubrick pleaded with him and he asked the screenwriter Terry Southern (who had been raised in Texas) to record a tape with Kong's lines spoken in the correct accent. Using Southern's tape, Sellers managed to get the accent right and he started acting in the scenes in the aircraft but then sprained his ankle and he could not work in the cramped cockpit set.
Slim Pickens, an established character actor and veteran of many Western films, was eventually chosen to replace Sellers as Major Kong after Sellers' injury. Terry Southern's biographer, Lee Hill, said the part was originally written with John Wayne in mind, and that Wayne was offered the role after Sellers was injured, but he immediately turned it down.[27] Dan Blocker of the Bonanza western television series was approached to play the part, but according to Southern, Blocker's agent rejected the script as being "too pinko".[28] Kubrick then recruited Pickens, whom he knew from his brief involvement in a Marlon Brando western film project that was eventually filmed as One-Eyed Jacks.[27]
His fellow actor James Earl Jones recalls, "He was Major Kong on and off the set—he didn't change a thing—his temperament, his language, his behavior." Pickens was not told that the movie was a black comedy, and he was only given the script for scenes he was in, to get him to play it "straight".[29]
Kubrick's biographer John Baxter explained, in the documentary Inside the Making of Dr. Strangelove:
As it turns out, Slim Pickens had never left the United States. He had to hurry and get his first passport. He arrived on the set, and somebody said, "Gosh, he's arrived in costume!", not realizing that that's how he always dressed … with the cowboy hat and the fringed jacket and the cowboy boots—and that he wasn't putting on the character—that's the way he talked.
Pickens, who had previously played only supporting and character roles, said that his appearance as Maj. Kong greatly improved his career. He later commented, "After Dr. Strangelove the roles, the dressing rooms, and the checks all started getting bigger.
https://en.wikipedia.org/wiki/Dr.Strangelove#Slim_Pickens_as_Major_T._J.%22King%22_Kong