The film, which will both star and be directed by Ben Stiller (his first directorial project since 2008's Tropic Thunder), is a remake of a 1947 film that featured Danny Kaye in the lead. That film was, in turn, based on the James Thurber short story first published in The New Yorker in 1939.
In the story, a henpecked husband (Stiller) copes with the frustrations of his dull life by imagining he is a fearless airplane pilot, a brilliant doctor, and more. Wiig would play a co-worker to Mitty and also an alternate version of that character in the fantasy sequences.
The current version of the film will feature a script from Steve Conrad, best known for The Pursuit of Happyness and The Weatherman.
Penn, who next appears in Ruben Fleischer's The Gangster Squad, would play the part of a photographer.