The teenager suffered “life-changing” injuries in the shooting near a foot tunnel in Hockley Circus shortly before 7pm on Thursday.
West Midlands Police said the area will remain closed on Friday as forensic inquiries are conducted.
The young victim was taken to hospital in a critical condition, the force added.
Police have said they are appealing for anyone with dashcam footage to come forward to help identify those responsible for the shooting.
Forensics are conducting a fingertip search underneath the busy flyover with a foot tunnel underneath closed from both ends.
The force also asked any witnesses or anyone with information to come forward.