April 30 - RCA owned NBC begins regularly scheduled television service from its New York station with the opening ceremonies of the 1939 New York World's Fair broadcast.
April 30 - Franklin D. Roosevelt becomes the first U.S. President to appear on television during the World Fair's opening ceremonies broadcast.
May 3 - The All India Forward Bloc is formed by Netaji Subhash Chandra Bose.
May 13 - The first commercial FM radio station in the United States is launched in Bloomfield, Connecticut. The station later becomes WDRC-FM.
May 14 - Lina Medina becomes the world's youngest confirmed mother in medical history at the age of five.
May 17 - The Columbia Lions and the Princeton Tigers play in the first-ever televised sporting event, a collegiate baseball game in New York City.
May 22 - World War II: Germany and Italy sign the Pact of Steel.
May 23 - The U.S. Navy submarine USS Squalus sinks off the coast of New Hampshire during a test dive, causing the death of 26 sailors. The remaining 32 crewmen and one passenger are rescued the following day.
May 27 - DC Comics publishes its second superhero in Detective Comics #27; he is Batman, one of the most topical comic book superheroes of all time.
June 1 - Maiden flight of the Focke-Wulf Fw 190 Würger (D-OPZE) fighter aeroplane
June 4 - Holocaust: The SS St. Louis, a ship carrying 963 Jewish refugees, is denied permission to land in Florida, United States, after already being turned away from Cuba. Forced to return to Europe, many of its passengers later died in Nazi concentration camps.
June 6 - German dictator Adolf Hitler gives a public address to returning German volunteers who fought as Legion Kondor during the Spanish Civil War.
June 12 - Shooting begins on Paramount Pictures' Dr. Cyclops, the first horror film photographed in three-strip Technicolor.
June 17 - Last public guillotining in France. Eugen Weidmann, a convicted murderer, is guillotined in Versailles outside the prison Saint-Pierre.
July 4 - Lou Gehrig, recently diagnosed with Amyotrophic lateral sclerosis, tells a crowd at Yankee Stadium that he considers himself "The luckiest man on the face of the earth" as he announces his retirement from major league baseball.
July 6 - Holocaust: The last remaining Jewish enterprises in Germany are closed.
August 2 - Albert Einstein and Leó Szilárd write a letter to Franklin D. Roosevelt urging him to begin the Manhattan project to develop a nuclear weapon.
August 15 - 13 Stukas dive into the ground during a disastrous air-practice at Neuhammer. No survivors.
August 23 - World War II: Germany and the Soviet Union sign a non-aggression treaty, the Molotov-Ribbentrop Pact. In a secret addition to the pact, the Baltic states, Finland, Romania, and Poland are divided between the two nations.
August 24 - The Molotov-Ribbentrop Pact (also known as Nazi-Soviet Pact) is signed between German foreign minister Joachim von Ribbentrop and Soviet foreign minister Vyacheslav Molotov.
August 26 - The first Major League Baseball game is telecast, a doubleheader between the Cincinnati Reds and the Brooklyn Dodgers at Ebbets Field, in Brooklyn, New York.
August 27 - First flight of the turbojet-powered Heinkel He 178, the world's first jet aircraft.
August 31 - Nazi Germany mounts a staged attack on Gleiwitz radio station, giving them an excuse to attack Poland the following day, starting World War II in Europe.
September 1 - World War II: Nazi Germany attacks Poland, beginning the war. (See Invasion of Poland.)
September 1 - George C. Marshall becomes Chief of Staff of the United States Army.
September 1 - The Wound Badge for Wehrmacht, SS, Kriegsmarine, and Luftwaffe soldiers is instituted. The final version of the Iron Cross was also instituted on this date.
September 1 - Switzerland mobilizes its forces and the Swiss Parliament elects Henri Guisan to head the Swiss Army (an event that can happen only during war or mobilization).
September 2 - World War II: following the invasion of Poland, Freie Stadt Danzig Danzig (now Gdańsk, Poland) is annexed by Nazi Germany.
September 3 - World War II begins when France, the United Kingdom, New Zealand, and Australia declare war on Germany after the invasion of Poland, starting the Allies.
September 4 - World War II: Japan declares neutrality in the European war.
September 5 - World War II: The United States declares its neutrality in the war.
September 6 - World War II: The Battle of Barking Creek.
September 6 - World War II: South Africa declares war on Germany.
September 10 - World War II: The submarine HMS Oxley is mistakenly sunk by the submarine HMS Triton near Norway and becomes the Royal Navy's first loss.
September 10 - World War II: Canada declares war on Nazi Germany, joining the Allies
September 17 - World War II: The Soviet Union joins Nazi Germany's invasion of Poland during the Polish Defensive War of 1939.
September 17 - World War II: A German U-boat U 29 sinks the British aircraft carrier HMS Courageous.
September 17 - Taisto Mäki becomes the first man to run the 10,000 metres in under 30 minutes, in a time of 29:52.6
September 18 - World War II: Polish government of Ignacy Mościcki flees to Romania.
September 18 - William Joyce's first Nazi propaganda broadcast.
September 21 - Romanian Prime Minister Armand Calinescu is assassinated by ultranationalist members of the Iron Guard.
September 28 - Nazi Germany and the Soviet Union agree on a division of Poland after their invasion during World War II.
September 28 - Warsaw surrenders to Nazi Germany during World War II.
October 1 - After a one-month Siege of Warsaw, hostile forces entered the city.
October 6 - World War II: The last Polish army is defeated.
October 8 - World War II: Germany annexes Western Poland.
October 14 - German U-Boat U-47 sinks British battleship HMS Royal Oak.
October 15 - The New York Municipal Airport (later renamed La Guardia Airport) is dedicated.
October 16 - World War II: First attack on British territory by the German Luftwaffe.
November 1 - The first rabbit born after artificial insemination is exhibited to the world.
November 4 - World War II: U.S. President Franklin D. Roosevelt orders the United States Customs Service to implement the Neutrality Act of 1939, allowing cash-and-carry purchases of weapons by belligerents.
November 6 - World War II: Sonderaktion Krakau takes place.
November 8 - Venlo Incident: Two British agents of SIS are captured by the Germans.
November 8 - In Munich, Adolf Hitler narrowly escapes the assassination attempt of Georg Elser while celebrating the 16th anniversary of the Beer Hall Putsch.
November 15 - In Washington, D.C., US President Franklin D. Roosevelt lays the cornerstone of the Jefferson Memorial.
November 17 - Nine Czech students are executed as a response to anti-Nazi demonstrations prompted by the death of Jan Opletal; in addition, Czech universities are shut down and over a thousand Czech students sent to concentration camps.
November 17 - The Rome-Rio de Janeiro air connection is created.
November 26 - Shelling of Mainila: The Soviet Army orchestrates the incident which is used to justify the start of the Winter War with Finland four days later.
November 30 - Winter War: Soviet forces cross the Finnish border in several places and bomb Helsinki and several other Finnish cities, starting the war.
December 2 - New York City's La Guardia Airport opens.