This was a fact not known until many years after WWII ended. After the attack on Pearl Harbor and when the US officially entered the second world war, the secret service determined that President Roosevelt needed a bulletproof car to protect him from possible assassins sympathetic to Japan or Germany.
At the time, Federal Law prohibited buying any cars that cost more than $750 (imagine that!), so they would have to get clearance from Congress to do that, and nobody had time for that. One of the Secret Service members, however, discovered that the US Treasury had seized the bulletproof car that Al Capone owned when he was sent to jail in 1931. They cleaned it, made sure it was running fine and had it ready for the President the day after.