Smallville’s version of the Flash was Bart Allen, instead of the most popular version of the DC Comics hero, Barry Allen. Barry Allen is the Scarlet Speedster that was used in CBS’ The Flash with John Wesley Shipp and in The CW’s Arrowverse, where the character is portrayed by Grant Gustin.
In Smallville season 4, Clark (Tom Welling) realized that there were other people out there who didn’t get their powers from the meteor rocks when he crossed paths with Bart Allen (Kyle Gallner). In his first episode, titled “Run,” Bart was a thief who could move so fast that he was capable of outrunning the Man of Steel himself. He left Smallville after a brief team-up with Clark, but returned in season 6’s Justice League episode where he served as a member of Green Arrow’s new team, along with Cyborg (Lee Thompson Young) and Aquaman (Alan Ritchson). He made another appearance in the season 8 finale when he and Black Canary (Alaina Huffman) showed up to help Clark and Oliver (Justin Hartley) defeat Doomsday.
Bart Allen was perceived as an interesting choice for Smallville, considering that he was the fourth incarnation of the DC Comics superhero, and a much less popular version of the hero in comparison to Barry Allen and Wally West. But there was a good reason for why Smallville used him and not the others. According to Smallville co-creator Al Gough, Bart is the Flash that DC approved, which indicates that both Barry and Wally were off-limits [via KryptonSite]. Based on the restrictions that DC has put on other characters in the past, this isn’t too surprising. Similar to the situation with Batman and Wonder Woman, DC had big plans for the Flash that didn’t involve a role on the small screen.
In the mid-2000s, Batman Begins writer David S. Goyer was working on a script for a Flash movie centered on Wally West. Supposedly, it would have been quite similar in terms of tone and direction to the Spider-Man movies that Sam Raimi directed. However, much like what happened with DC’s Wonder Woman plans at the time, the movie never materialized.
Though DC didn’t want the Flash on Smallville, they did ultimately allow the use of Bart Allen, albeit with some restrictions. While the Flash’s name was used in the episode promo, it was avoided in the show. Smallville writer Steven S. DeKnight has explained in the past that DC asked them not to call him the Flash, so the codename “Impulse” was used in its place. However, as DeKnight has noted, this didn’t stop viewers from seeing him as the Flash [via Killer Movies]. Smallville also made fun references to his comic book identities, and even included Easter Eggs of the other characters.
In Bart’s first episode, he was shown to be carrying fake ID’s with names such as Barry Allen, Wally West, and Jay Garrick. This was the extent at which these characters were used, but Bart proved to be an adequate substitute for them, as his rebellious attitude created an interesting contrast with Clark in season 4, and how his character matured into season 6 reflected the influence Clark’s good nature had on him.