Led by Robert Downey, Jr. as Dr. Doom, the superhero flick will bring together characters from 'The Avengers,' 'Fantastic Four' and the 'X-Men' franchise

"This is starting over from scratch," said Joe Russo at a SXSW London talk on Tuesday. "We want to make sure everybody feels like this isn't leaning on anything from the past."

Directors Joe and Anthony Russo are now referring to the upcoming film as 'Phase Zero.'