Palmerston Road, just off Wealdstone High Street, was blocked off with police tape and manned by uniformed officers.
A grey adidas sweatshirt with blood stains on the hood lay crumpled on the ground outside a Specsavers store along with a police exhibit number.
A shopkeeper said the youngster was "lucky to be alive" and they believe a bullet grazed the back of his head.
The manager of a betting shop, who did not give his name, said he was passing by on his lunch break and saw people gathered outside Specsavers around a black man on the ground.
He said: "He was holding his head down. I could not see his face but could see his white T-shirt was proper covered in blood.
"He was sitting calmly as the paramedics were looking after him."
He described Wealdstone as a "kind of rough area and it can be a bit aggressive".
He added that "seeing police around here is just another day" and he was used to seeing boys "just hanging around" in the street.