That it was meant to be the culmination of Leone's career is obvious from the title, a clear echo of Once Upon a Time in the West , and when we recall as well the fact that Leone favored Once Upon a Time...
Once Upon a Time in America, with its name obviously derived from Leone’s previous Once Upon a Time in the West, is a gangster film of the highest order, and, at the same time, it recalls so many of its predecessors, from the Warner Brothers classics of the 1930s to The Godfather.
Sergio Leone was the man who reinvented the western with the Dollars films starring Clint Eastwood and also the masterly Once upon a Time in the West.
Like Once Upon a Time in the West, Sergio Leone's 3-hour-and-47-minute gangster epic (1984) is a foundation myth, though the quality of the myth is very different: the focus here is individual rather than collective, and the form is cyclical and subjective rather than linear and expansive.
That honor will always belong to his other fairy tale-titled pastiche, Once Upon a Time in the West.
While filming “Once Upon a Time in the West,” Leone became enamored with “The Hoods,” a novel by “Harry Grey”, a gangster-turned-informant named Harry Goldberg whom Leone later befriended.