WASHINGTON—U.S. military officers in Afghanistan have drawn up preliminary proposals to withdraw as many as 5,000 troops from the country in July and as many as 5,000 more by the year's end, the first phase of a U.S. pullout promised by President Barack Obama, officials say.

The proposals, prepared by staff officers in Kabul, are likely to be the subject of fierce internal debate in the White House, State Department and Pentagon—a discussion influenced by calculations about how Osama bin Laden's death will affect the Afghan...