For the last 30 years at the Pentagon, October has been a season of change. Every few autumns, the military’s top officer cycles out and retires, bringing in new leadership and ideas.

President Obama is expected to announce his choice for the next chairman of the Joint Chiefs of Staff any day. And the selection could be a rarity: a Marine or Air Force general.

For 24 of the last 30 years, the chairman has been either a Navy admiral or Army general. Those officers include towering figures like Army Gen. Colin Powell, who was chairman during the Gulf War in 1991, and Adm. Mike Mullen, who led the Pentagon through the tail end of the Iraq War and the surge in forces in Afghanistan.

In those three decades, there has been just one officer each from the other services: Air Force Gen. Richard B. Myers and Marine Gen. Peter Pace. They served back-to-back in the first years after the Sept. 11, 2001, terrorist attacks, during the tumultuous period when Donald Rumsfeld was defense secretary.

Myers, as this 2005 piece by Washington Post columnist David Ignatius notes, was seen by some as too deferential to Rumsfeld. Pace replaced him in October 2005, but lasted just two years, rather than four like most JCS chairmen. As this Newsweek piece reported, Defense Secretary Robert Gates decided in 2007 that it would be best to start fresh after the Rumsfeld era with a new chairman.

That’s the backdrop now as Obama and new Defense Secretary Ashton B. Carter consider who should be their top general during the president’s waning time at the White House. Pentagon officials said there are four main candidates: Air Force Gen. Mark Welsh, the chief of staff of the Air Force; Marine Gen. Joseph F. Dunford, the commandant of the Marine Corps; Adm. Samuel J. Locklear III, the outgoing commander of U.S. Pacific Command; and Adm. James “Sandy” Winnefeld, the vice chairman of the Joint Chiefs since August 2011.

CNN reported this week that the top contenders are Welsh and Dunford, either of whom would represent a rarity modern history. Welsh, a fighter pilot by trade, would be just the second Air Force general to serve as chairman since Gen. David C. Jones held the job under presidents Jimmy Carter and Ronald Reagan. Dunford, an infantryman who led the war in Afghanistan from February 2013 until August 2014, would be the second Marine ever to hold the job.

Overall, half of the military’s 18 Joint Chiefs chairmen have been Army officers, including Gen. Martin Dempsey, who has held the post for nearly four years. Four have been admirals, four have been Air Force generals.