Does the NFL have a minor league?
League system
Gridiron football does not operate on a league system. Different professional leagues play by very different sets of rules in different seasons (the NFL plays 11-a-side on a 100-yard field in autumn and early winter, the CFL uses 12-a-side on a 110-yard field in summer and early fall, while arena football and the minor indoor leagues each play 8-a-side on a 50-yard field in the spring and early summer). There have been attempts at forming true minor leagues for the professional game (most recently with 2017's The Spring League); none so far have been able to balance the major leagues' requests with the ability to maintain financial solvency.
American Football League
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 World Championship Game 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.