Horrified witnesses described seeing “a lot of blood” as they did what they could to help the 16-year-old victim.
Police, paramedics and an air ambulance were scrambled to the scene in Kingstanding, to the north of the city, shortly after 2.30pm today.
An eyewitness told Birmingham Live the knife narrowly missed the victim’s heart.
They said: “Everyone helped him and we all got towels, there was a lot of blood.”
Janet Hart, 73, who lives on the road where the attack took place, said: “One ambulance was here for an hour, he looked in a bad way. It’s shocking.
“I really hope he’s going to OK. I’m praying for him.”
West Midlands Police said the boy’s injury is not thought to be life-threatening.
A force spokesman said: “A 17-year-old boy has been arrested after another teenager was stabbed on a bus in Kings Road, Kingstanding, at around 2.40pm today (Thursday 8 November).”
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