Well, technically Brad Pitt will play a four-star general "patterned after" General Stanley McChrystal in War Machine, a satirical comedy written and directed by David Michôd. While that distinction may be there to soften the blow of the comedy, the logic falls apart when you add in the fact that the movie is based on Michael Hastings' notorious book The Operators, with in turn was based on "The Runaway General," Hastings' even more notorious Rolling Stone profile of McChrystal, the one credited with ending the General's military career.
Got all that? Brad Pitt will play a general who probably won't be named "Stanley McChrystal" in a movie based on a book that was definitely written about General McChrystal. The movie will aim to be funny in a way that usually pisses off the people that it's about. Michôd previously wrote and directed Animal Kingdom, an excellent movie about Australian organized crime.
The other big news is the studio that's paying for the film: Netflix has acquired rights to the movie and will premiere it next year on the streaming service. There will probably be a brief theater run in New York & Los Angeles to make it eligible for awards, but you should be able to stream it pretty much right away. Pitt is the biggest movie star yet to put out a high-profile movie as a streaming-first release. The world's definitely changing.