In 2001, Justice League launched on Cartoon Network to almost universal acclaim. Building off of Batman and Superman solo animated series and spinning off into Justice League Unlimited, the creatives behind the show lead by Bruce Timm were also creating a blueprint that box-office busting superhero movies and TV universes are still using today. Over a decade before The Avengers officially took over the big screen, Timm and co. set a high bar for superhero team-ups. But they also had a secret.