The 13-year-old is believed to have been shot in the back while walking in the Hockley Circus area shortly before 7pm on Thursday.
He was rushed to hospital with injuries described as "life-changing" and has been in a critical condition since.
Armed police were called after that attack, and the subway under Soho Hill was soon cordoned off.
The Hockley Circus roundabout remained closed on Friday morning as forensic officers examined the scene.
No arrests have been made, but detectives are appealing for anyone who was in the area and may have information to come forward.
The roundabout is on a key route into the city from the Black Country and includes the Hockley flyover where the A41 meets the A4540 ring road just north of the Jewellery Quarter.
A West Midlands Police spokesman said: "Our investigations are in the very early stages, but we understand the 13-year-old was walking down Hockley Circus when he was shot in the back.
"We are urgently appealing to anyone in the area with any information. This is a busy area with lots of motorists, so we’re asking anyone with dashcams to check their footage as this may help us identify the people responsible.
"Also if you witnessed anything, please contact us as this information could provide integral to our investigation."
Anyone with information is asked to contact the police quoting log number 3589 of 18/11 or to call Crimestoppers anonymously on 0800 555 111.