Updated at 11:19 a.m.
President Obama named Ashton Carter, a former No. 2 Pentagon official, as his pick to succeed Defense Secretary Chuck Hagel.
Obama described Carter today as one of the "nation's foremost national security leaders."
"He was at the table in the situation room. He was by my side navigating complex security challenges that we were confronting," Obama said. I relied on his expertise and I relied on his judgment."
Carter called the nomination an "honor and a privilege."
He said he accepted the offer because of "the seriousness of the strategic challenges we face, but also the bright opportunities that exist for America if we grab hold of them."
He said, if confirmed, he will give Obama "candid" strategic and military advice. And, in a message to the U.S. military, he said, "I pledge to keep faith with you and to serve our nation with the same unflinching dedication that you demonstrate every day."
Carter's name began to surface this week as a possible replacement for Hagel, who announced Nov. 24 that he would step down once a successor is confirmed. NPR's Eyder Peralta noted that Carter, though unknown to the public, is "regarded as having a great intellect."
He is expected to enjoy bipartisan support during the nominating process.
Sen. Jim Inhofe, R-Okla., said earlier this week that he supports Carter "very strongly."
If confirmed, Carter will be Obama's fourth defense secretary (after Robert Gates, Leon Panetta and Hagel).
A Rhodes scholar, Carter has a doctoral degree in theoretical physics from Oxford University. He would inherit the Pentagon as the U.S. faces many global challenges, including the fight against the Islamic State militant group in Iraq and Syria, a resurgent Russia and unrest in other parts of the world. He also faces newer challenges such as cyberthreats.