In 2008, Rotunda signed a developmental contract with WWE and was assigned to Florida Championship Wrestling (FCW), where he debuted in December by defeating Caleb O'Neal. On December 16, 2008, Rotunda, wrestling under his real name teamed with Kris Logan to defeat Ryback and Jon Cutler in a tag team match. Throughout early 2009, Rotunda continued wrestling in tag team matches, and changed his name to 'Tank Rotunda' in February. In June, he began wrestling as 'Bo Rotunda' and later 'Bo Rotundo', and the following month he began teaming with his brother, Duke Rotundo. At the FCW television tapings on June 23, The Rotundo Brothers defeated The Dude Busters (Caylen Croft and Trent Barretta) to become the number one contenders to the FCW Florida Tag Team Championship. That same night, they defeated Justin Angel and Kris Logan for the FCW Florida Tag Team Championship. They went on to successfully defend the championship against Dylan Klein and Vance Archer and the team of Curt Hawkins and Heath Slater. At the FCW television tapings on November 19, the Rotundos lost the championship to The Dude Busters (Hawkins and Croft).
In August, FCW Florida Heavyweight ChampionMason Ryan issued an open challenge, which Rotundo accepted. He lost the subsequent championship match by submission. On February 3, 2011, Rotundo defeated Ryan to win the FCW Florida Heavyweight Championship for the first time, but lost the championship to Lucky Cannon immediately afterward. He regained the championship from Cannon three months later, on May 19. Rotundo made his WWE debut on July 25, defeating Primo in a dark match before the WWE Raw broadcast began. On August 16, Rotundo defeated Joey Ryan in another dark match at the SmackDown tapings, and the following week on August 23, Rotundo lost to Wade Barrett in a dark match prior to SmackDown. On September 1, Rotundo vacated the FCW Florida Heavyweight Championship due to injury. Later that month, it was reported that Rotundo had suffered a lacerated kidney, and was expected to be absent for several months to recuperate. Rotundo returned at the tapings of the January 29, 2012, episode of FCW television, cutting a promo declaring his intentions to reclaim the FCW Florida Heavyweight Championship. On February 2, Rotundo and his brother, renamed Husky Harris, defeated Brad Maddox and Eli Cottonwood, a substitute for the injured Briley Pierce, to win the FCW Florida Tag Team Championship for the second time. The brothers held the championship until March 15, when they lost it to Corey Graves and Jake Carter. In May 2012, Rotundo made several appearances at WWE house shows, defeating Drew McIntyre.
At a FCW live event on June 16, 2012, Dallas defeated Rick Victor to win the FCW Florida Heavyweight Championship for a third time, right after Victor had just defeated Seth Rollins for the title. He lost the title back to Victor at another FCW live event on July 13, 2012.
When WWE rebranded its developmental territory, FCW, into NXT Wrestling, Dallas' NXT television debut took place on the first episode of the rebooted WWE NXT taped at Full Sail University, as he played a happy-go-lucky and resilient babyface (heroic character), while defeating Rick Victor. On the August 8 NXT, Dallas was inserted into the Gold Rush Tournament to crown the first NXT Champion, where he was eliminated by Jinder Mahal in the quarter-finals. On the November 7 NXT, Dallas competed in a number one contender fatal four-way elimination match against Mahal, Drew McIntyre and Justin Gabriel; despite eliminating McIntyre and Gabriel, Dallas succumbed to Mahal. The next week, Dallas goaded Mahal into facing him in a match with the title shot on the line, but Mahal prevailed once again.
On January 26, 2013 at WWE's first-ever Royal Rumble Fanfest, Dallas participated in an eight-man NXT tournament and defeated Luke Harper, Conor O'Brian and lastly Leo Kruger to win the tournament, thus becoming the first NXT wrestler to qualify for the 2013 Royal Rumble match. The next day, Dallas entered the Rumble match at number 16, where he lasted over 20 minutes and eliminated the Intercontinental Champion Wade Barrett, who returned to the ring and eliminated Dallas. This started a feud between the duo, with Barrett challenging Dallas to a non-title match the following night on Raw, in which Dallas pulled off an upset victory in his first singles match on Raw. Dallas later challenged Barrett on the April 24 NXT for the Intercontinental Championship, but was defeated.
Simultaneously with his feud with Barrett, Dallas entered the NXT Tag Team Championship tournament alongside Michael McGillicutty, who had earlier saved Dallas from a beating at the hands of Primo and Epico. The pairing defeated Primo and Epico in the first round of the tournament. In the tournament semi-finals, Dallas and McGillicutty lost to the Wyatt Family (Luke Harper and Erick Rowan) due to Bray Wyatt's interference. An attack by the Shield then prevented Dallas from participating in a NXT Championship number one contender match against Conor O'Brian and Corey Graves (both who were later attacked as well). The triple-threat match did occur the next week and Wyatt prevented Dallas from winning the match due to Dallas' earlier refusal to join the Wyatt Family, resulting in a match being set up between Dallas and Wyatt, which Dallas won.
From late April 2013, the NXT audience turned against Dallas (despite Dallas not changing his character) and started booing him.NXT Tag Team ChampionAdrian Neville chose Dallas to replace his injured partner, Oliver Grey. On the May 8 NXT, Neville and Dallas failed in their first defense of the tag titles when they lost them to the Wyatt Family. Soon after, Dallas' character began showing subtle heel (villainous) traits of being disingenuous. On the May 29 NXT, Dallas competed in an 18-man battle royal to determine the number one contender to the NXT Championship; he eliminated his former tag partner Neville to win the battle royal. On the June 12 NXT (originally taped on May 23), Dallas defeated champion Big E Langston after twice ramming Langston into an exposed turnbuckle to become the new NXT Champion. As Leo Kruger set his sights on Dallas' title, Dallas made his first successful title defense on the July 3 NXT by exploiting Kruger's interference to defeat Antonio Cesaro. On the August 7 NXT, Dallas defeated Kruger to retain his title. By then, the NXT audience's hate of Dallas translated to them literally turning their backs on him while cheering his opponents, even heels like Cesaro and Kruger or unknowns like Mickey Keegan.
Dallas then held an open challenge to determine the next title contender but banned Sami Zayn from competing, so Zayn masqueraded as the masked El Local and defeated Dallas to earn his title shot. The match occurred on the October 16 NXT; General Manager John "Bradshaw" Layfield restarted the match when Zayn won despite Dallas' foot on the ropes, but not when Dallas sent Zayn into an exposed turnbuckle to retain his title. Dallas then faced his next title contender and former tag team partner Adrian Neville and lost by count-out to retain his title. However, Dallas would lose the title to Neville at NXT Arrival in a ladder match. He lost his rematch on the March 27 NXT. On the April 10 episode of NXT, Dallas attempted to mimic Daniel Bryan by trying to "Occupy NXT", but the crowd turned their backs on him as a response. He was then forced into a match against Justin Gabriel, which he won. Two weeks later he lost to Tyson Kidd, and after the match Bo threw a tantrum, mimicking what happened after the "Occupy NXT" incident weeks prior. On the May 8th edition of NXT, he participated in a number-one contender's Battle Royal, and made it to the final four before being eliminated by Tyson Kidd. On the May 15th NXT, it was announced that he was put in a match for the May 22nd show against Big E with the stipulation that if he won, he would earn another NXT title shot and if he lost, he would be forced to leave NXT forever. Dallas subsequently lost the match and was forced to leave. During the taping's break, he turned heel towards NXT (and only NXT) by saying that he hates everyone and that they should leave. Sooner in the NXT tapings for June, Bo secretly trespassed briefly under a new gimmick, "Mr. NXT". However, he lost to Sami Zayn who exposed Dallas and had security take him away. The following week after, he found his way into the building and ran around for help from security.
Since the April 7 episode of Raw, WWE started airing motivational-style introductory vignettes for Dallas. Dallas' return was in a winning effort, as he defeated Sin Cara on the May 23 episode of SmackDown as well as the May 26 episode of Raw. Dallas made his first pay-per-view appearance at Payback, where he was scheduled to face Kofi Kingston. However, the match never happened due to interference from Kane.  Dallas would face Kingston the following night on Raw in a winning effort. Following his début on the main roster, Dallas would go on an undefeated streak, picking up wins over the likes of R-Truth, Fandango, Titus O'Neil, El Torito, Damien Sandow, and The Great Khali. Dallas established himself as a heel when he attacked El Torito after his match against Diego on the July 4 episode of SmackDown. At Battleground, Dallas participated in the 19-man battle royal for the vacant Intercontinental Championship, he managed to eliminate Sin Cara and Titus O'Neil before being the 16th person eliminated after being thrown out by Dolph Ziggler. Bo Dallas was defeated on the July 28 edition of Raw by R-Truth, ending his undefeated streak at 17-1, prompting Dallas to do a post match assault after his loss. Four days later on SmackDown, Dallas would lose again to R-Truth via disqualification which prompted another assault on R-Truth.
On February 19, 2012, Rotunda was arrested for driving under the influence in Tampa, Florida, with blood-alcohol levels of 0.166 and 0.178 according to two breathalyser tests, more than twice the legal limit. He was released later that day on a $500 bond.