A security guard had petrol thrown in his face by robbers wielding a handgun before they escaped with an unknown quantity of money.
Police said three masked men snatched a delivery box from the guard after throwing fuel at him at Lloyds TSB in Cradley Heath, in the Black Country.
The gang then opened the container while they were inside the bank.
They then escaped in a stolen green Nissan Almeira car, driven by a fourth man, which was later found abandoned.
A spokesman for West Midlands Police said no shots were fired during the incident, which happened at 1230 BST on Tuesday, and that the Securicor guard was shaken but not seriously injured.