This fourth version of the AFL was the most successful, created by a number of owners who had been refused NFL expansion franchises or had minor shares of NFL franchises. The AFL's original lineup consisted of an Eastern division of the New York Titans, Boston Patriots, Buffalo Bills, and the Houston Oilers, and a Western division of the Los Angeles Chargers, Denver Broncos, Oakland Raiders, and Dallas Texans. The league first gained attention by signing 75% of the NFL's first-round draft choices in 1960, including Houston's successful signing of college star and Heisman Trophy winner Billy Cannon.
While the first years of the AFL saw uneven competition and low attendance, the league was buttressed by a generous television contract with the American Broadcasting Company (ABC) (followed by a contract with competing National Broadcasting Company (NBC) for games starting with the 1965 season) that broadcast the more offense-oriented football league nationwide. Continuing to attract top talent from colleges and the NFL by the mid-1960s, as well as successful franchise shifts of the Chargers from L.A. south to San Diego and the Texans north to Kansas City (becoming the Kansas City Chiefs), the AFL established a dedicated following. The transformation of the struggling Titans into the New York Jets under new ownership further solidified the league's reputation among the major media.
As fierce competition made player salaries skyrocket in both leagues, especially after a series of "raids", the leagues agreed to a merger in 1966. Among the conditions were a common draft and a championship game played between the two league champions first played in early 1967, which would eventually become known as the Super Bowl.
The AFL and NFL operated as separate leagues until 1970, with separate regular season and playoff schedules except for the championship game. NFL Commissioner Pete Rozelle also became chief executive of the AFL from July 26, 1966, through the completion of the merger. During this time the AFL expanded, adding the Miami Dolphins and Cincinnati Bengals. After losses by Kansas City and Oakland in the first two AFL-NFL National Championship Games to the Green Bay Packers (1967/1968), the New York Jets and Kansas City Chiefs won Super Bowls III and IV (1969/1970) respectively, cementing the league's claim to being an equal to the NFL.
In 1970, the AFL was absorbed into the NFL and the league reorganized with the ten AFL franchises along with the previous NFL teams Baltimore Colts, Cleveland Browns, and Pittsburgh Steelers becoming part of the newly-formed American Football Conference.
During the 1950s, the National Football League had grown to rival Major League Baseball as one of the most popular professional sports leagues in the United States. One franchise that did not share in this newfound success of the league was the Chicago Cardinals — owned by the Bidwill family — who had become overshadowed by the more popular Chicago Bears. The Bidwills hoped to relocate their franchise, preferably to St. Louis, but could not come to terms with the league on a relocation fee. Needing cash, the Bidwills began entertaining offers from would-be investors, and one of the men who approached the Bidwills was Lamar Hunt, son and heir of millionaire oilman H. L. Hunt. Hunt offered to buy the Cardinals and move them to Dallas, where he had grown up. However, these negotiations came to nothing, since the Bidwills insisted on retaining a controlling interest in the franchise and were unwilling to move their team to a city where a previous NFL franchise had failed in 1952. While Hunt negotiated with the Bidwills, similar offers were made by Bud Adams, Bob Howsam, and Max Winter.
When Hunt, Adams, and Howsam were unable to secure a controlling interest in the Cardinals, they approached NFL commissioner Bert Bell and proposed the addition of expansion teams. Bell, wary of expanding the 12-team league and risking its newfound success, rejected the offer. On his return flight to Dallas, Hunt conceived the idea of an entirely new league and decided to contact the others who had shown interest in purchasing the Cardinals. He contacted Adams, Howsam, and Winter (as well as Winter's business partner, Bill Boyer) to gauge their interest in starting a new league. Hunt's first meeting with Adams was held in March 1959. Hunt, who felt a regional rivalry would be critical for the success of the new league, convinced Adams to join and found his team in Houston. Hunt next secured an agreement from Howsam to bring a team to Denver.