B-52 bombers are capable of carrying nuclear weapons.
The air drill came ahead of combined large-scale exercises including amphibious landings starting later this month.
North Korea has traditionally called for those joint exercises to be called off, branding them as a prelude to invasion.
They have in the past drawn sharp reactions from Pyongyang including missile tests and nuclear threats, and North Korea's foreign ministry on Sunday demanded an immediate halt to U.S. - South Korea combined military drills, saying they were raising tensions.
With denuclearization talks stalled, North Korea conducted a record number of missile launches last year. As South Korea has lifted anti-Covid measures, the allies are returning to large-scale drills.
South Korea and the United States will achieve "peace through strength" by stepping up joint drills, the ministry in Seoul said in a statement.