Brad Bird was in part inspired to make this film as a memorial to his sister Susan, who died at the hands of her estranged husband by gun violence. His pitch was this: "What if a gun had a soul and didn't want to be a gun?"
Excluding the yells and groans, The Iron Giant only says a total of 53 words.
The 1999 film is based on a novel "The Iron Man". The author of the novel, Ted Hughes (who bears the same name as the characters Annie and Hogarth Hughes), wrote the novel as a way of comforting his children after the suicide of their mother Sylvia Plath.
The last name for Annie and Hogarth, Hughes, is an homage to Ted Hughes, author of the original children's book upon which the film is based.
The scene the Giant's hand is in the living room watching a Maypo cereal advertisement was changed in the "Signature Edition" to an advertisement for the Disneyland attraction, Tomorrowland. Brad Bird originally wanted the Tomorrowland advert in the film as an homage to his mentors, but Disney would not give him clearance to use it. Bird ended up directing the film Tomorrowland - Il mondo di domani (2015).
Brad Bird: [A113] The "3" on the A-113 license plate on the car, partially eaten by the Giant, is bitten off. Also, in Dean's house there is a painting which has "A113" on it. (A113 (sometimes A-113 or A1-13) is an inside joke, an Easter egg in animated films created by alumni of California Institute of the Arts, referring to the classroom used by graphic design and character animation students.)