WASHINGTON (Reuters) - The U.S. Navy would save $5.2 billion through fiscal 2017 by buying 10 fewer Boeing Co P-8A Poseidon maritime surveillance planes under the Pentagon's proposed budget for fiscal 2013, according to Pentagon budget documents.

"Due to changing priorities within the department and funding constraints, the department deemed that it was a manageable risk to reduce P-8A procurement by 10 aircraft from fiscal 2013 to fiscal 2017," the Pentagon's comptroller said in an overview of the budget.

The Navy's plans call for unchanged procurement of 13 P-8A Boeing planes in fiscal 2013 and 17 in fiscal 2014, but would cut the buy to 20 planes each in fiscal 2015 and 2016, dropping to 13 in the final year of the five-year budget plan.

The Navy's fiscal 2012 budget had called for the Navy to buy 21 of the Boeing planes in 2015 and 30 in 2016, according to a Navy spokeswoman.

The Navy is replacing its aging P-3 Orion fleet with the new Boeing planes, which are based on its 737 commercial airliner.

The Navy said the program remains a priority, noting that the P-8A's ability to launch torpedoes from a high altitude and its intelligence capabilities make it a "critical force multiplier" for military commanders.

The Navy expects to start using the new planes operationally in 2013, according to its budget documents.

The Navy's top budget official, Rear Admiral Joseph Malloy, said the Navy planned to add the deferred planes back into the Navy budget in fiscal 2018 and beyond.

No comment was immediately available from Boeing.

(Reporting By Andrea Shalal-Esa; Editing by Maureen Bavdek, Gerald E. McCormick and Bernard Orr)