Guardians of Justice is rendered and painted by the artists of DC Comics. It is inspired by the Warner Bros. Television Animation series The Justice League Unlimited. This series comprises an expanded Justice League which now includes Green Arrow, Captain Atom and Supergirl. The series is produced by award-winning Warner Bros. producers Bruce Timm and Glen Murakami
The original Justice League was conceptualized by longtime DC Comics’ writer, Gardner Fox. He was inspired by the Justice Society to create a similar, contemporary concept to the Justice Society of America, but one utilizing the word "league." Part of Fox’s concept was based on the teams that comprised the National Basketball League and the American Baseball League.
Characters: Martian Manhunter ,Green Lantern, Wonder Woman, Superman, Batman, The Flash, Hawkgirl (standing) and The Red Tornado, Zatanna, Green Arrow, Aquaman, Supergirl, Dr. Fate, Wildcat and Black Canary (flying)
Animator Bruce Timm, who co-produced Batman: The Animated Series and Superman: The Animated Series in the 1990s, became executive producer on an animated series focusing on the Justice League. The roster consisted of Batman, Superman, Wonder Woman, Green Lantern (John Stewart), The Flash (Wally West), Martian Manhunter (J'onn J'onzz), and Hawkgirl.
According to audio commentary on the DVD release of Season 2, the second season finale "Starcrossed" was expected to be the final episode of the series. However, in February 2004, Cartoon Network announced a follow-up series, Justice League Unlimited, which premiered on July 31, 2004 and featured a larger roster of characters.
Kevin Conroy reprised his voice role as Batman from Batman: The Animated Series (1992–1995), The New Batman Adventures (1997–1999), and Batman Beyond (1999–2001). Batman's costume was redesigned, but this time, his costume was a combination of his last three costumes. The same costume from The New Batman Adventures is retained, but with the blue highlights from the Batman: The Animated Series costume and the long-ears from the Batman Beyond costume are added to the costume. Tim Daly, who voiced Superman in Superman: The Animated Series (1996–2000) was initially involved but was unable to continue his role due to involvement with The Fugitive (a short-lived remake of the original 1963 TV series), and was replaced by George Newbern. Superman was initially redesigned to have a bit of a squint to his eyes and slight wrinkles that was also meant to make him look older, in addition to having a noticeable shining streak to his hair; he was redesigned to appear larger in physical girth than in the previous series. Fans did not like the older appearance and in the second season the streak was toned down to the point of almost disappearing and the squint was removed, in essence reverting Superman to his earlier animated look. As an in-joke, Superman's season one facial designs are used for an older Jor-El in the Justice League Unlimited episode "For the Man Who Has Everything".