Here's a short (translated) extract from a Sergio Leone's autobiography.

Sergio Leone :
"This movie was born out of our love for the American cinema and literature of the 1920s and 1930s. It could be called 'Once Upon A Time There Was A Certain Film Genre'. It is the portrait of America as shown by the Hollywood cinema that fascinated us. The main character is the sum of what all the other gangster films heroes have done before.
When I met the author of the book (Philip Goldberd, a Jewish gangster locked in Sing-Sing, wrote "The Hoods"), an ex-con, he told me that he had written, among other thgs, against Hollywood cinema, but what particularly struck me in his book was the similarity with Hollywood films. It is a perfect example of the way imagination takes over from reality. It is also an intimist movie which follows the souvenirs of a man. It may be the first time ever that a movie ends on a flashback. Hence the main character could have imagined his whole future thanks to opium, it could only be a journey in an imaginary world. I am generally fascinated by flashbacks because it allows you to tell a whole life story with very few images and also because it makes the story more mysterious and fascinating."

Franco Ferrini (one of the script writers) :
"Leone wanted to make a gangster movie but he wanted his mobsters to be Jewish, because he wanted to avoid the comparison with the Italian mobsters from the 2 Godfather films.

Right from the start the script took the opposite way of the story in the book. The novel is written by an insider but he lacks imagination. We all were aware that we didn't tell the history of gangsterism but his mythology revisited by European eyes."