This is a Grosser Mercedes 770 K; W150 Staatskarosse, one of Hitler's armoured cars.
The car was captured in 1945 by American soldiers at Hitler's mountain villa at Berchtesgaden and a few years later it was brought to Canada.
The G.I. soldiers who discovered the car had heard it was armored with "bullet proof" windows and so they wanted to test the windows to see if this was true.
One of the soldiers pulled out his pistol and fired several shots at the windows only to learn that the rumor was true, the windows did not break. Source