The flight will likely anger North Korea, which is extremely sensitive to the deployment of high-tech U.S. weapons systems.
The United States often sends powerful warplanes to South Korea in times of heightened animosity between the Koreas, but it is still unusual for such aircraft to fly near the rivals' border, the world's most heavily fortified.
The United States flew two B-1B bombers over South Korea yesterday.
The US Pacific Command said the flight was the closest a B-1 has ever flown to the border. It didn't say whether both or just one of the bombers approached the border.