Lt. Col. Chris Logan said the operation was part of US Pacific Command's "Continuous Bomber Presence" missions, which he said are "intended to maintain the readiness of US forces."
"US Pacific Command's CBP missions, which have been routinely employed since March 2004, are flown in accordance with international law," he added.
The flyover came shortly after Mattis used a Saturday speech in Singapore to accuse China of "intimidation and coercion" in the region and declared that the United States does not plan to abandon its role there.
"Make no mistake: America is in the Indo-Pacific to stay," Mattis said. "This is our priority theater."
Mattis specifically called out Beijing's militarization of artificial islands in the South China Sea, home to some of the world's busiest sea lanes. "We are aware China will face an array of challenges and opportunities in coming years, we are prepared to support China's choices if they promote long-term peace and prosperity for all in this dynamic region," Mattis said.
The Pentagon last week ratcheted up rhetoric about China's militarization of islands in the South China Sea, even as the Trump administration presses China for cooperation on North Korea.
When asked by a reporter about the ability of the US to "blow apart" one of China's controversial man-made islands, Lt. Gen. Kenneth McKenzie, director of the Joint Staff, told reporters, "I would just tell you that the United States military has had a lot of experience in the Western Pacific taking down small islands."
His comments - a reference to US amphibious landings and capture of Japanese held islands during World War II - come amid growing tension in the hotly contested region, as the US ramps up freedom of navigation operations in response to China's steady militarization of its artificial islands.
The Chinese government has reacted furiously to the recent US statements. At her regular press conference on Thursday, Chinese Foreign Ministry spokeswoman Hua Chunying said the US accusing China of militarizing the region was, "like a thief crying, 'Stop thief!'."
"Why does the US choose to sail every now and then close to Chinese South China Sea islands and reefs? What is the US trying to do?" she said.