Affleck, who is also set to play Batman in Superman reboot sequel, is reteaming with Warner Bros for the film.
The 41-year-old actor and filmmaker will both star in and direct the project, based on a script by Will Staples, reported Ace Showbiz.
Affleck's friend and collaborator Matt Damon is producing the film with Jennifer Todd under Pearl Street Films.
The film is "an examination of the moral ambiguities of how philanthropy and foreign assistance veers into modern day neocolonialism" as it follows a group of hired mercenaries set to kill a warlord who has been victimising his own people.
LeHane wrote Affleck's 'Gone Baby Gone'.
