The 21-year-old started the year with Class A Visalia, and after compiling a 1.26 ERA in five starts, he was promoted to Double-A Mobile.
In 21 games for the BayBears, Bradley was 12-5 with a 1.97 ERA and helped pitch Mobile into the Southern League Championship Series.
Drafted out of high school with the No. 7 overall pick in the 2011 First-Year Player Draft, Bradley does not have to be added to the 40-man roster this year. However, if the team brought him to the big leagues, he would need to be added immediately.
"To add him to the roster in the winter when roster spots are very valuable to us -- and the team being pretty much out of contention -- didn't make a lot of sense," Towers said.
Bradley showed better command this year, cutting down his walks from 84 in 136 innings last year to 69 in 152 innings this year.
"I think he still, based on what I saw and our scouts, he needs to use his secondary pitchers a little bit more," Towers said. "Really refine the curveball and the changeup."
While Bradley was able to dominate with his fastball in the Minors, Towers said he'll need to be able to use his secondary pitches to get big league hitters out.
The team plans to make Bradley a non-roster invitee to big league camp next spring and allow him to compete for a spot in the Opening Day rotation.
"I think he deserves it," Towers said.
Steve Gilbert is a reporter for MLB.com. Read his blog, Inside the D-backs, and follow him on Twitter @SteveGilbertMLB. This story was not subject to the approval of Major League Baseball or its clubs.