When development started on the show in 2008, there was concern about launching the show with Captain America as the leader of the team. A great deal of research showed that Cap met a lot of negativity outside of the United States. So, to give people a chance to know the character, it was decided to have Iron Man, whose popularity skyrocketed at the time of development due to the Iron Man (2008) film, lead the team. Most importantly they wanted to mirror the original Stan Lee & Jack Kirby run and form the Avengers using the original 5 members and have Cap join later after they discover him in ice.
In terms of tone and style, the series is based primarily on the original stories by Stan Lee and Jack Kirby. However, it also utilizes material from all eras of the comic's run and other sources such as the Marvel Cinematic Universe.
The show debuted on Disney XD in Fall 2010 starting with 20 micro-episodes (which are actually the first 5 episodes broken down) as a way to capture that feeling from the comics in Avengers #1 and The Avengers (2012) live action film where you see Super Heroes with established worlds and adventures of their own embarking on a Super Hero team-up of epic proportions.
Christopher Yost decided to change Captain Marvel's appearance for this series in order to create a renewed interest in the character. Although this Captain Marvel is Mar-Vell, he has some aspects of the Ultimate version, like his full name and rank, abilities, and human alias.
The producers loosely considered the live action films Iron Man (2008) and The Incredible Hulk (2008) to be Iron Man and Hulk's background for the series. For the Hulk there was the stipulation that the final scene in that movie is what gets the Hulk/Banner relationship to where it is and a more intelligent incarnation of the Hulk. Also, the Abomination has a British accent as he did in the film.
The opening sequence features the original 5 founding members and Captain America fighting HYDRA and ends with the Avengers characters featured in the episode doing a pose together. For Season 2 the intro was altered and featured Nick Fury narrating. However, since episode 20 of Season 1, Disney XD started using the Season 2 intro for the remaining Season 1 episodes.
Although HYDRA appeared to be a stand-in for Nazi Germany, Christopher Yost would later clarify they were a branch of the Nazi regime. That is the same way HYDRA would be portrayed in Captain America: The First Avenger (2011).
For Season 2 many of Avengers characters get new costumes. Iron Man got a new red and gold armor, Thor's new costume is the Olivier Coipel version that inspired the Thor movie in turn, the Skrull that impersonates Captain America gets a new uniform that is more like the ultimate version plus a new energy shield, the Hulk gets a pair of shorts that stretch with each transformation, and Black Panther gets a cape. Ironically Wasp from the comics is a huge fashionista and yet somehow one of the only characters in the show that hasn't gotten a Season 2 makeover.
The series initially features a team based on the roster for the original Avengers, composed of Iron Man, Thor, Ant-Man, the Wasp, and the Hulk. This due to the first comics appearance of The Avengers in The Avengers #1 (Sept. 1963).
Wasp's costume is an original look by character designer Ciro Nieli inspired by her two most iconic and memorable costumes: her original Red & Black costume and her gold and black costume. Wasp's costume features a pattern that has 'W' for Wasp across the front, which comes to a stinger in the back.
This project originally began as "Incredible Hulk: Gamma Corps" which was going to feature a combination of Hulk's Warbound and the hit squad from the Gamma Corps comic mini-series. The show was also going to feature Hulk teaming up with other super heroes. With scripting for pre-production underway, it was felt that the creative work that was being done to bring Hulk's world and his villains to life was too good to relegate to his universe alone, and would be much better suited as part of an full-on Avengers adaptation.
Season 2 was supposed to premiere in Fall 2011, but it was decided to change the premiere to April 2012 to promote interest for the upcoming Avengers movie and to be paired with Ultimate Spider-Man on Disney XD's Marvel Universe block.
The lost episode "Powerless" was not aired on Disney XD because it progressed a storyline with Surtur that was not going to be further explored since the series would not get a third season. It premiered on the Season 2 part 1 DVD in 2012 and was not aired on Disney XD until 2013.
The theme song heard in the opening is called "Fight as One" and features a verse to describe the main 4 Avengers (Hulk, Captain America, Thor, and Iron Man). In the full song there's a second verse to describe the remaining 4 Avengers.
When picking a costume for Ms. Marvel, it was decided to use her first costume the red and blue costume with the scarf and starburst on her chest. Because it was her first costume in the comics, producer Josh Fine thought it would be appropriate to introduce her in that costume.
Christopher Yost was more or less transitioned off the show at the mid-season point with episode 39, but worked on a few more episodes, including the final episodes of the season, along with Jeph Loeb and Man of Action who took over running the show.