President Barack Obama could be close to nominating the first-ever woman to become the head of a military combatant command, Pentagon sources tell NPR.

The U.S. military divides the world into areas of responsibility run by four-star generals and admirals, but none has ever been female. Obama wants to change that before the end of his term, Pentagon sources say, by naming a woman to take command of U.S. Northern Command, which also runs the well-known North American Aerospace Defense Command, or NORAD.

NORAD and NorthCom are charged with defending the U.S., including scrambling Air Force fighters to respond to the recent intrusions by Russian bombers near American airspace. There's talk that its current commander, Navy Adm. Bill Gortney, could be retiring before his term is up at the end of next year, which would create the opportunity to make history with a female commander.

Two names on the shortlist are Air Force Gen. Lori Robinson, who now commands U.S. Air Forces in the Pacific, and Adm. Michelle Howard, who today serves as the vice chief of the Navy.

Robinson was the first woman to lead a major Air Force component command, and served as a senior air battle manager aboard an E-3 Sentry airborne early warning aircraft and the E-8 JSTARS — essentially responsible for command and control of combat missions, as well as surveillance.

Howard, meanwhile, is the number two officer in charge of running the day-to-day operations of the Navy. She made her way into popular culture when she ran the Navy's anti-pirate task force off East Africa back in 2009 and was portrayed (at least over the radio) in the movie "Captain Phillips," ordering a Navy destroyer to respond to a hijacking by Somali pirates.

Howard is also the Navy's first female four-star admiral; NPR profiled her last year.

Meanwhile, other combatant commands are coming open, including Central Command, responsible for running operations in Syria, Iraq and Afghanistan.

There's an expectation that the current commander in Afghanistan, Gen. John Campbell, will take that spot next spring when Gen. Lloyd Austin steps down. But some sources point to the current commander of U.S. Special Operations Command, Gen. Joe Votel. That would be an unusual move: An officer switching from one combatant command to another.

Special Operations Command's top job could go to Lt. Gen. Tony Thomas, who now runs the Joint Special Operations Command at Fort Bragg, N.C., or Lt. Gen. Kenneth Tovo, who runs the U.S. Army Special Operations Command at Fort Bragg.

Then there's Africa Command. Its current boss, Gen. David Rodriguez, steps down in the spring. Marine Lt. Gen. Thomas Waldhauser has been talked about for that job, although it could go to General Campbell if he doesn't get Central Command.

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