The bombers were dispatched from Andersen Air Force Base in Guam to the peninsula, where they were joined by South Korean and American fighter jets.
The 10-hour mission came three days after North Korea tested an intercontinental ballistic missile.
Two B-1B Lancers reminded North Korea Friday that the USA and its allies are prepared to unleash unbelievable firepower in the event of a renewed conflict on the Korean Peninsula.
Moon said it is unclear whether North Korea is capable of mounting a nuclear warhead on its long-range missile, though Seoul and Washington are continuing to treat the latest developments very seriously.
The U.S. bombers conducted a live fire drill over a shooting range in Gangwon Province, along with two of South Korea's F-15K fighter jets and two U.S. F-16 combat planes.
On Wednesday, the USA and South Korean militaries fired simultaneous ballistic missiles in a simulated attack against the DPRK, as a "strong message of warning".
An article in state-owner newspaper Rodong Sinmun said the drill "justified" North Korea's nuclear weapons tests. During a 20-minute speech, Moon laid out the policy directions for achieving permanent peace on the Korean peninsula amid heightened global and regional tensions after Pyongyang's test of an intercontinental ballistic missile on Tuesday.
Tension has been high as the USA administration under President Donald Trump and the North's regime under leader Kim Jong-Un have exchanged hostile rhetoric for months.
Based on North Korea's claim and the Pentagon's assessment that the rocket poses a new level of threat to the United States and the region, the U.S. ambassador to the UN dedicated the entire UNSC meeting to scrambling together a new set of sanctions to impose on Pyongyang. The penetrate takes after a jointly mounted guns and rocket practice by South Korean and US powers a day after the North's ICBM test.
"Something has to be done about it", Trump said as he met with Chinese President Xi Jinping.
The US, Japan and South Korea agreed on Friday to push for a quick UN Security Council resolution to put new sanctions on North Korea.
The B-1B bombers dropped inert weapons to practice their attack capabilities. Trump the same day warned of a "severe response" to the latest missile test.
North Korean leader Kim Jong-un visited a mausoleum honouring state founder Kim Il-sun on Saturday, the anniversary of his grandfather's death, the North's official KCNA news agency reported.