EDITOR'S NOTE: This article is part of a series, "Countdown to Endgame," which looks back at the films of the Marvel Cinematic Universe leading up to Avengers: Endgame.

With two massively successful Iron Man films and a moderately successful Hulk reboot under its belt, Marvel Studios began development on the most ambitious projects of its early years with its next three films: Thor, Captain America: The First Avenger and The Avengers.

The first film would feature a mythological figure blending the worlds of science fiction and fantasy that eschewed traditional conventions in mainstream superhero films, while the second would be almost entirely a period piece set during the height of World War II starring the least cynical superhero in the entire Marvel Universe. Upon the new characters' introduction, the studio would then deliver on its longstanding promise to include them all in the most ambitious cinematic crossover at that time.

For a studio whose first film was only four years old, this was an extraordinary risk, the concept of the Marvel Cinematic Universe little more than a novelty with cameo appearances and Easter eggs at that point. It also meant wider genre shifts, with Thor tenuously featuring the MCU's first serious exploration in sci-fi and fantasy, while Captain America was a war film, just one that featured a star-spangled hero wielding an indestructible shield. For a shared cinematic universe still in its relative early stages, it was a bold move, especially as behind the scenes troubles began to plague Thor.

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Marvel Studios had originally signed Matthew Vaughan to helm the first Thor film, though the filmmaker ultimately left the project over budget concerns and directed X-Men: First Class instead. While Guillermo del Toro was approached, he similarly opted to write and direct The Hobbit before being himself being replaced by Peter Jackson. It was in classically trained Kenneth Branagh that Marvel Studios finally found its Thor director, with the release date pushed from July 2010 to May 2011 to accommodate the delays.

After Daniel Craig turned down the title role due to his commitments to the James Bond franchise, Branagh cast relative unknown actors Chris Hemsworth and Tom Hiddleston as Thor and his adopted brother, Loki, while filling the supporting cast with more established actors, including Natalie Portman, Anthony Hopkins and Rene Russo, to lend the film more gravitas.

thor and loki 2011

In the promotional campaign leading up to the film's release, the cast and crew downplayed the more mythological, fantasy elements of the character. Instead, Asgard and its inhabitants were referred to as aliens from a faraway world that visited Earth millennia ago and inspired Norse myths rather than actually being gods themselves, with their advanced technology indistinguishable from magic to more primitive civilizations.

To help sell the out-there concept of the property, much of Thor is a fish out of water story with comedic flourishes as the Prince of Asgard is cast out by his father Odin to Earth, where he learns humility while growing close to humanity through Jane Foster and her colleagues. After dealing with Loki's inevitable betrayal, Thor would prove himself worthy of regaining his powers and vow to protect Earth whenever it needed him.

NEXT PAGE: Thor and Captain America Set the Stage for an Unprecedented Ensemble

Thor proved to be a hit with audiences, garnering a positive critical response and earning $449.3 million at the worldwide box office. Two months later, Captain America: The First Avenger would open in theaters everywhere, ushering in the last MCU hero to set the stage for the following year's crossover.

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The first Captain America film had been in development for some time and saw previous MCU directors Jon Favreau and Louis Letterier approaching Marvel Studios to helm the project before the production signed Joe Johnston based on his work on the World War II pulp hero film The Rocketeer. While initially reluctant to be cast in the title role due to its contractual commitments for five further films, going as far to turn down an offer three times, Chris Evans ultimately signed on to portray Captain America.

Set almost entirely in the 1940s, the film shows the creation of Captain America to combat Hydra, a rogue Nazi war machine led by the Red Skull. As the earliest film chronologically in the MCU, The First Avenger not only introduces Steve Rogers and his supporting cast, including Bucky Barnes, but also showcases the precursor to S.H.I.E.L.D. while building up the importance of the Tesseract, first introduced in the post-credits scene for Thor.

A sequence of Captain America awakening in the present day was moved to the end of the main film while a sizzle reel for The Avengers was included after the credits to build anticipation for the ensemble crossover. While not as commercially successful as Thor, Captain America: The First Avenger became the third highest earning World War II film, behind Saving Private Ryan and Pearl Harbor, and was second only to the original Iron Man as the most critically acclaimed MCU film on Rotten Tomatoes at the time.

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All of this steady development would finally pay off the following year with filmmaker Joss Whedon at the helm of The Avengers. First signed in April 2010, Whedon would completely rewrite the script originally penned by Zak Penn, picking up directly from the events of the first Thor and Captain America: The First Avenger.

As the actors from previous MCU films began to assemble, the production faced a major casting hurdle: It lacked a Hulk. Marvel Studios did not extend Norton's initial contract to reprise his role as the Jade Giant, candidly citing a refusal by the actor to be a willing participant in the collaborative process. Ultimately, Mark Ruffalo was cast as the new Bruce Banner, joined by Cobie Smulders as Maria Hill, with filming lasting from April to September in 2011.

Following the formation the team after Loki returns to Earth to obtain the Tesseract under orders from a mysterious benefactor revealed to be Thanos, The Avengers was a colossal hit. Earning over $1.5 billion and the best critical response to the MCU since Iron Man, the film was the highest earning superhero film in history at the time and led to rival studios scrambling to develop their own shared cinematic universes. The MCU's future burned brighter than ever having accomplished a promise first made by the studio in 2008. But just as the first phase of the MCU was spent building up its heroes, the second phase would focus on tearing them down.