The bombers will take part in the drills at a time of heightened tensions on the Korean peninsula and as United Nations political affairs chief Jeffrey Feltman makes a rare visit to North Korea.
The North has warned the drills would push the Korean peninsula to "the brink of nuclear war".
Some analysts and diplomats hope Feltman's visit to the North could spark a U.N.-led effort to defuse rising international tensions over Pyongyang's nuclear and missile programmes.
North Korea's state media confirmed the arrival of Feltman and his entourage late on Tuesday without offering more details.
Feltman, a former senior U.S. State Department official, is the highest-level U.N. official to visit North Korea since 2012. The U.S. State Department said on Tuesday he was not carrying any message from Washington during his visit.
The drills, which started on Monday and will run until Friday, come after North Korea tested last week what it called its most advanced intercontinental ballistic missile (ICBM) that could reach all of the United States.
South Korean government officials, including President Moon Jae-in, have said the North's latest ICBM is their most capable yet, although it has several critical points to prove, such as re-entry technology and terminal stage guidance.
A South Korean defence ministry official was unable to immediately verify the Yonhap report about the U.S. B-1B Lancer bombers taking part in the drills on Wednesday.
Aircraft taking part will be flown from eight U.S. and South Korean military installations.
The U.S. Air Force has said the size of this year's drill is "comparable" to previous years.
North Korea has vehemently criticised the drills since the weekend, saying the exercise "precipitates" the United States' and South Korea's "self-destruction".
China and Russia had proposed that the United States and South Korea stop major military exercises in exchange for North Korea halting its weapons programmes. Beijing formally calls the idea the "dual suspension" proposal.
Russia has communication channels open with North Korea and Moscow is ready to exert its influence on Pyongyang, RIA news agency quoted Russian Deputy Foreign Minister Igor Morgulov as saying on Tuesday.
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