May 1: The breakfast cereal Cheerios is introduced as CheeriOats by General Mills. - Orson Welles' film Citizen Kane premieres in New York City. - The first Defense Bonds and Defense Savings Stamps go on sale in the United States, to help fund the greatly increased production of military equipment.
May 2: Anglo-Iraqi War: British combat operations against the rebel government of Rashid Ali in the Kingdom of Iraq begin.
May 5: Emperor Haile Selassie enters Addis Ababa, which has been liberated from Italian forces; this date is subsequently commemorated as Liberation Day in Ethiopia.
May 6: At California's March Field, entertainer Bob Hope performs his first USO Show.
May 8: The German auxiliary cruiser Pinguin is sunk by HMS Cornwall (56) in the Indian Ocean.
May 9: The German submarine U-110 is captured by the British Royal Navy. On board is the latest Enigma cryptography machine, which Allied cryptographers later use to break coded German messages.
May 10: The British House of Commons is damaged by the Luftwaffe in an air raid. - Rudolf Hess parachutes into Scotland, claiming to be on a peace mission.
May 12: Konrad Zuse presents the Z3, the world's first working programmable, fully automatic computer, in Berlin.
May 15: The first British jet aircraft, the Gloster E.28/39, is flown. - Joe DiMaggio's 56-game hitting streak begins as the New York Yankee center fielder goes one for four against Chicago White Sox Pitcher Eddie Smith.
May 19: The Viet Minh is formed in at Pác Bó in Vietnam to overthrow French rule of the nation as an alliance between the Indochina Communist party, led by Ho Chi Minh, and the Nationalist party.
May 20: The Battle of Crete begins as Germany launches an airborne invasion of Crete.
May 21: German submarine U-69 sinks the U.S.-flagged SS Robin Moor off the west African coast, having allowed the passengers and crew to disembark.
May 24: In the North Atlantic, the German battleship Bismarck sinks battlecruiser HMS Hood, killing all but 3 crewmen from a total of 1,418 aboard the pride of the Royal Navy.
May 24: The British submarine HMS Upholder torpedoes and sinks the Italian ocean liner SS Conte Rosso.
May 26: In the North Atlantic, Fairey Swordfish aircraft from the carrier HMS Ark Royal cripple the steering of Bismarck in an aerial torpedo attack.
May 27: Franklin D. Roosevelt, President of the United States, proclaims an "unlimited national emergency." - German battleship Bismarck is sunk in the North Atlantic, killing 2,300. - The Swiss Socialist Federation is banned.
May 30: Manolis Glezos and Apostolos Santas tear down the Nazi swastika on the Acropolis in Athens, and replace it with the Greek flag.
May 31: Anglo-Iraqi War: British troops complete the re-occupation of the Kingdom of Iraq, returning Prince 'Abd al-Ilah to power as regent for Faisal II. (source: Wikipedia)