According to Den of Geek, after the success of Richard Donner’s Superman: The Movie, Warner Bros. decided it was Batman’s turn for the big screen treatment.
Mankiewicz’s Batman was similar in tone to Donner’s Superman, and would have been an origin story for the Caped Crusader.
Other details include that it would have had multiple villains (Joker and Penguin) and that it had a big Bond-esque set-piece for the climax.
One of the possibilities to direct the film was Joe Dante, who was coming off hot after Gremlins.