The thieves targeted the Royal Bank of Scotland, in Main Street, Cleland, near Motherwell in North Lanarkshire, shortly after 2pm yesterday.
The armed men, who drove a black Volkswagen Golf GTI, pulled up next to a G4S security van which was delivering money to the bank.
One of the men got out of the car and followed a security guard into the bank where he threatened him with a gun.
The man then demanded a case containing an unknown amount of cash.
After being handed the case, the man ran out of the bank to the waiting car.
The pair sped off in the direction of Shotts.
The car was later found burnt out on nearby road and forensic tests are being carried out on the vehicle.
No one was injured in the robbery, however, customers and staff were left shaken.
Detective Chief Inspector Pat Campbell, who is leading the investigation, appealed for witnesses or anyone with information to get in touch.
He said the only description of the two men is that they were wearing dark clothing and masks.
Mr Campbell said: "It's imperative we catch the men responsible for this robbery.
"Brandishing a firearm in broad daylight is totally unacceptable and the consequences could have been much more serious.
"It is possible the car that was used may have been in the area prior to the robbery, and I would appeal to anyone who saw it to contact us.
"We are checking CCTV and already, a number of people have come forward. Anyone who has any information should contact CID at Wishaw on 01698 202 600 or Crimestoppers on 0800 555 111."