Animator Bruce Timm, having successfully adapted both Batman and Superman into animated television programs in the 1990s, took on the challenge of faithfully adapting the Justice League comic book. Ignoring the sidekicks, pets and other extraneous elements of the earlier Super Friends show, the line-up of this new JLA adaptation was created with two things in mind: to pay tribute to the original line-up of the Justice League of America while also reflecting racial and cultural diversity. Significantly, the well-known (but much-deprecated) superhero Aquaman was left out of the lineup (although he would be used on the show) in favor of a second female on the team - Hawkgirl - and the African-American Green Lantern John Stewart, who has worked with the League in the comics before, was used rather than either of the better-known modern-era Green Lanterns Hal Jordan and Kyle Rayner, even though Rayner had appeared as Green Lantern in the Superman animated series. (In the second season, Rayner is described as a Lantern in training under Stewart's old mentor, explaining his absence. Both he and Jordan make brief appearances in Justice League Unlimited.)
The show met with significant success, partially due to loyal fans already familiar with these incarnations of the characters, and partially from a new generation of viewers. The two-part nature of most episodes led Cartoon Network to choose to air the episodes back-to-back.
In February 2004 Cartoon Network announced a follow-up series, Justice League Unlimited, which premiered on July 31. Justice League Unlimited features a greatly expanded roster of heroes, usually with only a few appearing in any given episode, although there are a few featuring just about the entire roster fighting against one giant enemy.
Tim Daly, who voiced Superman in his previous animated series, did some early recordings, but ultimately he was unable to reprise the role due to his involvement with the short lived remake series of The Fugitive, resulting in his being replaced by George Newbern. Newbern was heavily criticized by fans during the first season, who felt that his Superman was too stiff and lacked both the everyman feel and heroic authority that Daly brought to the role, which Bruce Timm said was more his fault and the fault of his staff than Newbern's and has openly defended Newbern. Since then, many fans have agreed that Newbern improved significantly during the second season and now have no problem with his portrayal of Superman, and some have noted that his grunts sound very similar to Daly's. 
Superman was initially redesigned to have a bit of a squint to his eyes that was also meant to make him look older, in addition to having a noticeable shining streak to his hair; additionally, he was redesigned to appear larger in physical girth than in his previous series. Fans did not like the older appearance and in the second season the streak was toned down the point of almost disappearing and the squint was removed.
Maria Canals was cast as Hawkgirl based on her Hispanic accent, which the producers felt would make Hawkgirl appear more foreign and alien against her team mates.
In the comic books, the Martian Manhunter/J'onn J'onzz has a power called "Martian Vision" which is essentially the same as Superman's heat vision, but the power has never been shown in the series, dropped presumably in favor of his phasing power and telepathy. The Martian Manhunter was only refered to by that name in the Justice League Unlimited episode "Task Force X" by the Clock King. Otherwise, he is never referred to as 'Martian Manhunter', only as J'onn.