The Concorde, which by creating a sonic boom it traveled through the atmosphere at supersonic speed, was built with funding from the British and French governments, began its conducted its first commercial flight on January 21st. In 2000, an Air France Concorde’s engine failed and the plain crashed killing all 109 people on board and 4 people on the ground. Many believe this event was key in the retirement of the Concorde in 2003.
Other notable events on January 21st:

In 1968, the Battle of Khe Sanh, one of the most publicized battles of the Vietnam War, commences.

In 1911,  the first Rally Monte Carlo takes place, with 23 cars taking part.

In 1793, Louis XVI, the last Bourbon king of France, was executed by guillotine during the French Revolution in Paris.

In 1506, Swiss Guards first arrived at the Vatican as watchmen for the pope.

The Concorde seen in the Technik Museum Sinsheim. Photographer: W. Bulach