December: Timely Comics' Captain America Comics #1 (cover dated March 1941), first appearance of Captain America and Bucky, hits newsstands in the United States.

December 1: Manuel Ávila Camacho takes office as President of Mexico.

December 6: British submarine HMS Regulus (N88) is sunk near Taranto.

December 8: The Chicago Bears, in what will become the most one-sided victory in National Football League history, defeat the Washington Redskins 73-0 in the 1940 NFL Championship Game.

December 9: Operation Compass - British forces in North Africa begin their first major offensive with an attack on Italian forces at Sidi Barrani, Egypt.

December 12 and December 15: "Sheffield Blitz" - The Yorkshire city of Sheffield is badly damaged by German air-raids.

December 14: British destroyers HMS Hereward (H93) and HMS Hyperion (H97) sink an Italian submarine off Bardia. - Royal Navy Fairey Swordfish based on Malta bomb Tripoli. - Plutonium is first synthesized in the laboratory by a team led by Glenn T. Seaborg and Edwin McMillan at the University of California, Berkeley.

December 16: Operation Abigail Rachel - RAF bombing of Mannheim.

December 17: President Roosevelt, at his regular press conference, first sets forth the outline of his plan to send aid to Great Britain that will become known as Lend-Lease.

December 23: Winston Churchill, in a broadcast address to the people of Italy, blames Benito Mussolini for leading his nation to war against the British, contrary to Italy's historic friendship with them: "One man has arrayed the trustees and inheritors of ancient Rome upon the side of the ferocious pagan barbarians."

December 24: Mahatma Gandhi, Indian spiritual non-violence leader writes his second letter to Adolf Hitler addressing him "My friend", requesting him to stop the war Germany had begun.

December 29: Franklin D. Roosevelt, in a fireside chat to the nation, declares that the United States must become "the great arsenal of democracy." - "Second Great Fire of London" - Luftwaffe carries out a massive incendiary bombing raid, starting 1,500 fires. Many famous buildings, including the Guildhall and Trinity House, are either damaged or destroyed.

December 30: California's first modern freeway, the future State Route 110, opens to traffic in Pasadena, California, as the Arroyo Seco Parkway (now the Pasadena Freeway). - In Sweden, Victor Hasselblad forms the Victor Hasselblad AB Camera Company. (source: Wikipedia)