Prince John's infantile behavior whenever someone mentions his mother is a sly reference to the real King John "The Fool" of England. The brother of King Richard "the Lionheart", John, the favorite son (and heavily spoiled) by their father King Henry II, was said to be crude and dim-witted, and attempted to usurp the throne from Richard while the latter fought in the Crusades. Though Richard eventually returned from battle and foiled John's rebellion, John eventually became king anyway after Richard's untimely death (1199) in battle without an heir.
A few months before the film's release, the Disney animators needed Peter Ustinov to come back to the Disney Studios to re-record some of his lines as Prince John. The animators made phone calls to New York, London, Paris, Vienna, and Tokyo, trying to locate Ustinov -- only to discover that he was working at the NBC Studios in Burbank that week, a half-mile down the street from them.
The characters of the Sheriff (Pat Buttram), Friar Tuck (Andy Devine), Nutsy (Ken Curtis), and Trigger (George Lindsey) were all voiced by actors known for doing westerns. This is because at one point in the production the setting was the Old West.
The University of Southern California fight song, "Fight On", is played during the chase scene after the archery contest. "On Wisconsin", the fight song of the University of Wisconsin, is also used during the "football" sequence of this scene.
Initially, the studio considered a movie about the European fable Reynard the Fox. However, due to Walt Disney's concern that Reynard was an unsuitable choice for a hero, Ken Anderson used many elements from it in Robin Hood. Particularly the animal counterparts (e.g. Robin Hood, like Reynard, was a fox and The Sheriff, like Isengrin, was a wolf.)
Tommy Steele was the original choice to perform the voice of the title character, but he was dropped out shortly after recording sessions began. Terry Jones of "Monty Python" fame was also considered for the role.
The respective voices of Maid Marian and Lady Kluck, Monica Evans and Carole Shelley, also worked together in the Previous Disney Animated film, The AristoCats (1970), as the voices of the Two Geese Abigail and Amelia.
The trivia item below may give away important plot points.
A possible ending included Robin Hood being wounded after escaping from the castle. In this ending Prince John, hooded and cloaked, is about to stab Robin but is interrupted by the arrival of King Richard. The filmmakers felt it made Prince John too sinister and took away from Robin Hood as the hero. It was also seen as too dark a turn from the otherwise upbeat and carefree tone of the film and so it was deleted and replaced with the ending where Robin Hood not only survives unscathed but Prince John, Sir Hiss and The Sheriff of Nottingham are imprisoned for their treachery. The original ending, in storyboard form, is included in the "Most Wanted Edition" DVD.