Downey most recently starred in the spring blockbuster "Iron Man" and will soon be seen in the big-screen comedy "Tropic Thunder."
The Hollywood Reporter said Imagine Entertainment partners Brian Grazer and Ron Howard are producing "Cowboys & Aliens," which is based on a graphic novel written by Fred Van Lente and Andrew Foley from an original idea by Platinum Studios chairman and Chief Executive Officer Scott Mitchell Rosenberg.
Rosenberg will also produce, along with DreamWorks bosses Steven Spielberg, Alex Kurtzman and Roberto Orci. Ervin Rustemagic and Rich Marincic will co-produce, the entertainment industry trade paper said.
Set in the American Old West, the movie is about how a battle between the Apache and Western settlers is interrupted when a spaceship crashes into the prairie near Silver City, Ariz., forcing the two factions of warring Earthlings to band together and fight a common enemy, The Hollywood Reporter said.
Downey would play a former Union Army gunslinger named Zeke Jackson in the film, which was written by "Iron Man" authors Hawk Ostby and Mark Fergus.