McCullough, 21, initially attended North Carolina, but was dismissed from the program last fall for a violation of an unspecified team rule. He then transferred to Lander, which is located in South Carolina. The Bearcats made their first NCAA Division II College World Series appearance this year.
Listed at 6-foot-4, 240 pounds, McCullough has a mid-90s fastball, but he has been said to have touched 100 mph.
"Certainly that type of velocity is not just running around," said D-backs scouting director Ray Montgomery of McCullough, who was scouted by George Swain. "Add to it that he's a big, strong kid. I feel pretty confident in turning him over to [player development] and letting them do their thing with them."
To go along with the blazing fastball, McCullough has a decent mid-80s slider that he does not use very much.
The biggest key for McCullough will be improving his control. This past season, he walked nearly a batter per inning and at times worked out of the stretch with no one on base in an attempt to throw more strikes.
While at North Carolina, McCullough pitched out of the bullpen where he profiles as a pro. He was 0-1 with a 0.73 ERA in 17 appearances for the Tar Heels in 2013.
Last year for Lander, he started 15 games and was 5-1 with a 4.59 ERA. He allowed just 37 hits in 64 2/3 innings, but he walked 61 while striking out 87.
"He's not a pure strike thrower," Montgomery said. "When you're throwing that hard at a young age, it's hard to control it. When we saw him against the wood bats in the summer in the Cape, he was really good."
The Draft concludes on Saturday, with exclusive coverage of Rounds 11-40 beginning on MLB.com at 10 a.m. MST.
Steve Gilbert is a reporter for MLB.com. Read his blog, Inside the D-backs, and follow him on Twitter @SteveGilbertMLB. Adam Lichtenstein is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.