On what story is Good Kill (2014) based?

Officially, Egan's fictional character is based on a composite of stories from drone pilots. Says the film's director:

When it says based on true events, what that means is that even though the pilot that Ethan Hawke plays is a composite character, every strike that you see has occurred. I did not make any of that. Nothing that you see happening was embellished. I spoke to drone pilots, in particular the case of the woman who he tries to rescue, they say seeing atrocities like that had nothing to do with their mission, and they cannot act to expose themselves, so they don't. But they have seen these atrocities and they would definitely like to act.

Unofficially, it might be based on the story of Brandon Bryant, though it's a touchy subject. Bryant critiqued an early version of the script and suggests the producers might've used his notes and later snubbed him:

I was thinking specifically of Brandon Bryant, the former Air Force drone operator who has become a de facto spokesperson for an invisible air force suffering from deep remorse over their kills, the bad ones and the good ones. A 2013 DoD study reported that drone operators can suffer from the same combat stress as aircraft pilots, including PTSD. Bryant says he was approached by the producers of Good Kill in 2013 and gave notes on an early script, but says they mostly dropped communications with him.

"Andrew Niccol took my story and warped it to his own," says Bryant, who has seen the film. "They snubbed me and created a terrible film with no intelligence behind it."

In fact, The Telegraph goes as far as saying the film was 'loosely based' on Bryant's experiences.