The spinoff, which aired as a backdoor pilot during "Chicago Fire's" first season, revolves around the Chicago Police Department's investigative team.
Bush, who starred in CBS' short-lived comedy "Partners" last season, will play Erin Lindsay, a member of the team with an unexpected past. She replaces Melissa Sagemiller, who was the original female lead in the pilot.
Flueger will play Kyle Ruzek, a hotheaded rookie plucked by Voight (Jason Beghe) from the Academy for dangerous, delicate undercover work for the Intelligence unit.
Bush, Flueger and Soffer were all attached to co-star in NBC's "Hatfields & McCoy" drama, which the network passed on in May.
"Chicago Fire" returns for Season 2 on Tuesday, Sept. 24 at 10/9 p.m. CT, while "Chicago PD" is slated to premiere in early 2014.
View slideshow: Meet the "Chicago PD" cast members