After directing The Good, the Bad and the Ugly, Leone decided to retire from Westerns and desired to produce his film based on The Hoods, which eventually became Once Upon a Time in America. However, Leone accepted an offer from Paramount to provide access to Henry Fonda and to use a budget to produce another Western film. He recruited Bertolucci and Argento to devise the plot of the film in 1966, researching other Western films in the process. After Clint Eastwood turned down an offer to play the villain's nemesis, Bronson was offered the role. During production, Leone recruited Donati to rewrite the script due to concerns over time limitations.
The original version by the director was 166 minutes (2 hours and 46 minutes) when it was first released on December 21, 1968. This was the version that was to be shown in European cinemas and was a box office success. However, for the US release on May 28, 1969, Once Upon a Time in the West was edited down to 145 minutes (2 hours and 25 minutes) by Paramount and it was greeted with a mostly negative critical response and was a financial flop. The film is now generally acknowledged as a masterpiece and one of the greatest films ever made.
In 2009, it was named to the United States's National Film Registry by the Library of Congress for being “culturally, historically or aesthetically” significant and will be preserved indefinitely.