- Feb 27 Martin Kamen and Sam Ruben discover carbon-14 (radiocarbon dating) at the University of California Radiation Laboratory in Berkeley, California
- Feb 29 Hattie McDaniel becomes 1st African American woman to win an Oscar for "Gone with the Wind"
- Mar 18 Benito Mussolini and Italy join Hitler in Germany's war against France & Britain
- May 10 Winston Churchill succeeds Neville Chamberlain as British Prime Minister
- May 12 Nazi blitzkrieg conquest of France began by crossing Muese River
- May 13 Winston Churchill says "I have nothing to offer but blood, toil, tears & sweat" in his first speech as Prime Minister to British House of Commons
- May 15 McDonald's opens its first restaurant in San Bernardino, California
- May 27 British and Allied forces begin the evacuation of Dunkirk (Operation Dynamo) during WWII
- Jun 4 British complete the "Miracle of Dunkirk" by evacuating 338,226 allied troops from France via a flotilla of over 800 vessels including Royal Navy destroyers, merchant marine boats, fishing boats, pleasure craft and even lifeboats
- Jun 14 Auschwitz concentration camp opens in Nazi controlled Poland with Polish POWs (approx. 3 million would die within its walls)
FCC hears 1st transmission of FM radio with clear, static-free signal
On January 5, 1940
Walt Disney begins serving as an informer for the Los Angeles office of the FBI; his job is to report back information on Hollywood subversives.
On November 10, 1940