Alex Gibney says David Ellison promised HBO will air his controversial Elon Musk documentary

Elon Musk.

Kevin Dietsch/Getty Images HBO will air Alex Gibney's Elon Musk documentary, "Musk," the filmmaker told Business Insider.

The nearly four-hour documentary delves into the life and career of the richest man in the world.

The air date is tentatively set for January, according to Gibney.

Alex Gibney's latest project, the Elon Musk documentary "Musk," is already in theaters.

But when news broke last week that David Ellison's Paramount had officially bought Warner Bros.

Discovery , the director admits he was "concerned" about the doc's future release on HBO.

"Musk," the unflinching and often unflattering nearly four-hour look at the richest man in the world, was produced by HBO, which is under the Warner Bros.

Discovery umbrella.

With Ellison, who once reportedly considered bringing Musk on as an equity investor in Paramount, now owning WBD, would he shut down plans to show "Musk" on HBO after its theatrical run? "I was concerned until recently," Gibney told Business Insider over a video chat on Wednesday.

"I've been told that they will air it and that even David Ellison very well knows about it and promises he will air it." Gibney added that, as of now, "very specific plans" are in the works for HBO to air "Musk" sometime in January.

HBO declined to comment; Skydance did not respond to a request for comment.

Alex Gibney.

Tiziana FABI / AFP via Getty Images Gibney's history with HBO goes back decades and has resulted in some of the Oscar winner's most popular works, including 2015's " Going Clear: Scientology and the Prison of Belief " and his 2019 Elizabeth Holmes documentary, " The Inventor: Out for Blood in Silicon Valley.