"Avengers: Age of Ultron" won't hit screens until May of 2015, but today (July 29) you can finally learn the truth about how "The Avengers" script was assembled in the first place in this exclusive peek at Joss Whedon's real story.

In "Joss Whedon: The Biography" -- which was penned by Amy Pascale, and will hit shelves via Chicago Review Press on August 1 -- we finally learn the truth behind the first draft of "The Avengers" script, as well as how and why Whedon got on board in the first place.

With behind-the-scenes details from the likes of Kevin Feige, Chris Hemsworth, and Whedon himself, this particular superhero director's origin story was well worth the wait:

“We needed somebody who wasn’t going to reinvent the wheel, because the die was cast and the cast was cast,” said Kevin Feige, president of production at Marvel Studios. “And yet we wanted somebody with a unique voice, because this had to feel like a part one, not Iron Man 3 or Thor 2.” Marvel also needed someone who could both handle action and juggle the multiple storylines required in an ensemble piece.

Feige was well aware that Joss Whedon was a strong contender. “I’ve known Joss for many, many years,” he says, “going back to almost ten years ago when we had a project set up at New Line back before Marvel was our own studio. We would license our properties [Spider-Man, the Fantastic Four] out to other studies to produce the films, and New Line had a movie that we were sort of interested in Joss directing. We went to lunch with him, and we brought him into New Line. As a very strange aside, the first meeting we were going to have with him we had to reschedule because it was September 11, 2001. And the project that we were talking about having Joss direct at New Line: Iron Man.”

According to Feige, Joss remained on Marvel’s radar for the next decade. He wrote a treatment for Iron Man that was approved, but with Buffy and Angel on the air and Firefly in development, he had to back out. There were also brief discussions with him to direct X-Men and X2 (both of which were ultimately helmed by Bryan Singer). More recently, he’d put in a good word with Thor director Kenneth Branagh when his Cabin in the Woods star Chris Hemsworth was auditioning for the title role. “I was about to go back for the audition for Ken and Joss called him just, you know, without me even knowing and just said, ‘Hey, look. I really like this guy’ and ‘He’s talented and fights for the right things,’” Hemsworth says. “Ken really respected his opinion, and I’m sure that helped me get the job.” (Joss and Drew Goddard also prepped the actor with a slew of Thor comic books.)

Joss, says Feige, “was always somebody that I was a very, very big fan of and I always sort of hoped that our paths would cross, but frankly, he’s got the Whedonverse, right? He’s got his own empire. But when I started making calls about The Avengers, his agent mentioned his name. And I said, ‘Of course.’”

Joss agreed to the meeting because he was a fan of the Avengers, but he didn’t necessarily want to take on the project. First, Dr. Horrible had proved that he could have great success on a project that he fully controlled. He and Kai were already set to meet with their accountant to do another self-funded project. Second, would he even be the right person to write for the Avengers? In 2005, he told In Focus magazine that the X-Men universe was the only Marvel property he felt comfortable in. “The thing about the X-Men is they have a coherent core. The Avengers to me is tough. I wouldn’t approach The Avengers, I wouldn’t approach the Fantastic Four,” he said. “The X-Men are all born of pain, and pain is where I hang my hat.” Third, he’d already struggled in Serenity to tell a cohesive story about a diverse group of people with extensive backstories that would be accessible to both devoted fans and complete newbies. And last, he’d been burned in his past three attempts to make a comic book movie, the perpetually delayed Wonder Woman project being the latest and most frustrating failure.

He went into the meeting with a single intention: to have a discussion about the current version of the script, by Zak Penn, the screenwriter for The Incredible Hulk. That is not what happened. Between the end of filming on Thor and the start of filming for Captain America, Joss and Feige got together, and the production chief explained the Avengers script and laid out how each movie related to the next.

“I don’t think you have anything,” Joss told Feige. “You need to pretend this draft never happened.” Then he went home and wrote five pages of material explaining how he would tell the story of the Avengers. “In the process of writing it, I got that bug,” Joss said. “I realized, oh, yeah, this would be so much fun.”

Joss’s document, according to Feige, was incredibly well written and articulate, was full of great ideas, and ended with the motto “The Avengers: Some Assembly Required.” “What a great way to kick off this project!” says Feige. He loved the phrase so much that when they added an Avengers teaser to the end of Captain America in July 2011, they used it as a tagline. “That’s where ‘Some Assembly Required’ came from—I remembered it from Joss’s original memo.”

It was the underlying concept of family that spoke to Joss. “He’s got a great quote,” Feige says, remembering Joss’s line about The Avengers being a group of people who have no business being in the same room together, and yet they are forced to work together. “Isn’t that the perfect definition of a family?”

Marvel liked Joss’s ideas and gave him an offer to write and direct, with several stipulations, including a ninety-two-day shoot and a quick turnaround for postproduction, even the substantial special effects sequences. The studio specified that the villain must be Loki, brother of the fallen Norse god Thor, who turns against his sibling in the superhero’s solo film. Also, it wanted a big fight among the Avengers in the middle of the film that leaves the team shattered, and an epic battle at the end. “I was like, great, you just gave me your three acts,” Joss said. “Now all I have to do is justify getting to those places and beyond them.” Or as he put it at another point, “I have enough signposts to build from, all I had to do was try to make it matter and try to have reasons for the conflicts.”

The Avengers was already set for release in May 2012, which gave Joss confidence that, unlike Wonder Woman, this film was really going to happen. He was all in.

“I kept telling my mom that reading comic books would pay off,” Joss joked. His stepfather, Stephen, had jokingly taunted him for years with the question “When are you going to make a real, grown-up picture, without the vampires and the rocket ships?” Each time, Joss would reply with the same answer: “Never. It’s never gonna happen.” Now he was about to board the biggest rocket ship of his career.