The Italian title of the film is
C'era una volta il west.
The original story was dreamed up (after weeks of the three watching the great American westerns) by Sergio Leone, Dario Argento, and Bernardo Bertolucci. Argento later became a director of stylish, if gory, horror films like Suspiria (1977) and Tenebrae (1982, heavily censored and released in the U.S. as Unsane). Bertolucci is best known for more artistic films like The Conformist (1970, Il Conformista) and the Oscar-winning The Last Emperor (1987).
From a 300 page treatment, Leone and co-writer Sergio Donati fashioned a script that treats the west and the western as grand opera, both creating and revising the myths of the genre (plus some added black humor). In an inspired move, the production filmed in Monument Valley, made famous by the classic westerns of John Ford (again an homage to the western and a backdrop to rework its conventions within their story).
Continuing his long relationship with the prolific Italian film composer Ennio Morricone, Leone had the score written prior to the film and even used it as an aid with the acting and pace of the scenes. Each of the four main characters was given his or her own musical theme with its own style. Bronson's "Man with a Harmonica" theme is especially chilling, which complements and is complemented by his acting.