The aircraft then carried out a series of bombing simulations against a military facility.
This military exercise was held a week after Pyongyang launched an intercontinental ballistic missile that is claimed to reach the US mainland.
The South Korean military said the US bombers took part in a series of bombing simulations involving US and South Korean fighters in Gangwon province, which borders North Korea.
More than 200 fighter planes and thousands of soldiers were involved in a military drill that ended on Friday and were planned before the North's latest missile launch.
Pyongyang, which routinely denounces joint US-South Korean military exercises, in its latest statement on Monday called it a "provocative act toward nuclear war".
The joint military exercise was held when UN political envoy Jeffrey Feltman visited Pyongyang for a dialogue, a rare visit by a senior UN official.
Tension on the Korean peninsula has been on the rise since a missile launch late last month, triggering international condemnation, including the US threat that the North Korean regime will be "completely destroyed" if war is really happening.
The South Korean military also plans to launch combat units using weapons aka "dronebots" next year, Yonhap news agency quoted an unnamed official as saying.
"Dronebots" will conduct reconnaissance with North Korean military force targets, and can also launch attacks, although the report does not provide more details about how it can be done.