The Flash's Barry Allen isn't the first live-action version of the Scarlet Speedster, or even the second, as nearly 15 years after the 1990 Flash TV series, Smallville brought Bart Allen to the small screen in 2004. Bart Allen became the first DC Comics character outside of the world of Superman to appear on the show.
Superman prequel series Smallville told the story of a young Clark Kent and his journey to become the Man of Steel. The show, which originally aired on The WB (before becoming The CW), famously had a "No Tights, No Flights" rule, which meant that Clark Kent couldn't suit up and fly as Superman until the very last scene of the series finale. Regardless, Smallville dug deep into Superman and DC Comics lore. Though Clark didn't become Superman until the very end, the show's ten-season run allowed him to experience some of Superman's most iconic stories and meet some of the most important people in the DC Universe.
Throughout the first three seasons, each episode generally dealt with Clark going up against a super-powered enemy who received their abilities from being infected by Kryptonite. Smallville season 4, episode 5, "Run", broke from the traditional format and introduced a character who didn't get his powers from Kryptonite. In this episode, Clark meets metahuman Bart Allen (Kyle Gallner), the fourth version of the Flash in DC Comics and the grandson of Barry Allen. In the show, Bart uses his super speed to live life to the fullest, without caring about the consequences of his actions. Clark tries to set Bart on the right path and keep him out of trouble.
A particular scene in the episode provides a fun reference to other, more recognizable versions of the Flash. When looking at Bart's fake IDs, Clark notices the names "Jay Garrick", "Barry Allen", and "Wally West". Also, Bart doesn't initially wear a costume, but he does keep the Flash's red color scheme. In his first appearance, his red backpack contains a yellow lightning bolt symbol. Clark tries to race Bart, but at the end of the episode Bart proves that he's the "Fastest Man Alive" by easily outrunning Clark. Bart eventually returns in-costume as a member of the Justice League, under the name "Impulse". In the comics, "Impulse" is Bart's original superhero identity.
Bart Allen's guest-starring role in season 4 was a game-changing moment for Smallville, as the character became the first of many DC Comics heroes to cross paths with Clark and his friends. A season 5 episode introduced Aquaman, and before long, Smallville began bringing the characters together for large scale team-up episodes that involved heroes like Bart Allen, Aquaman, Martian Manhunter, Green Arrow, Cyborg, and more. Green Arrow even became a series regular. So for Smallville, Bart Allen was the beginning of something huge.