After getting offered the chance to star in I magnifici sette (1960), McQueen found out that the only way he could do the film, which was being shot simultaneously with "Wanted: Dead or Alive", was to fake an accident or illness and get a medical leave from the series. According to his first wife, Neile, McQueen accomplished this feat by "faking" a car crash in which he merely crashed his car into a tree, receiving minor cuts, muscle pulls, and bruises, and getting his medical leave. The series' production went on temporary hiatus while McQueen filmed "The Magnificent Seven".
Steve McQueen's characterization of a sympathetic bounty hunter was first tested in an episode of the Western series, Trackdown (1957) which starred Robert Culp as a Texas Ranger.
The Mare's Leg weapon carried by Josh Randall (Steve McQueen) is a cut down Winchester model 1892 carbine in 44-40 caliber, but the bullets in his cartridge belt are 45-70 caliber rounds used in the larger, more powerful rifles of the day. The producers wanted to use the 45-70s because they were more visually impressive than the relatively small, pistol sized rounds actually used in the 1892 carbine. The use of the 1892 carbine is itself an anachronism as the series is set in the 1870s.
Initially, the creators of the series had a hard time selling the show because bounty hunters were thought to be unsavory characters and have been portrayed as such in numerous western shows and movies. The creators overcame this obstacle by having Josh Randall give most, sometimes ALL, of his earnings to help people such as the families of people murdered by the men Randall brings in, thus making Randall a sympathetic and likable character.
Steve McQueen was hired after Jack H. Harris, who was producing Fluido mortale (1958) gave him a glowing reference to Dick Powell (the head of Four Star Productions). Powell also asked for, and was granted, the opportunity to view a rough cut of that film.