The men, believed to be Asian, pointed the firearm at the driver in St Andrew's Road, Small Heath, on Monday.
The offenders made off with about £60 in cash before police arrived on the scene within minutes of the 999 call at 3.50am.
Officers searched the surrounding streets but found there was no sign of the offenders.
The driver wasn't injured in the incident.
Detectives have seized CCTV and police enquiries are under way, with anyone with information urged to contact the force.
Contact West Midlands Police by using the live chat function on their website or by calling 101.
Quote crime reference 20BW/10368G/21.