Story highlights Efforts to curb costs could leave the U.S. on the hook for hundreds of untested aircraft

The Pentagon's testing office questioned the strategy and legality of buying the fighters in bulk

Washington (CNN) The military risks committing itself to buying billions of dollars of F-35 fighter jets before they have demonstrated they are fit for combat, a new Pentagon report warns.

The development of the Joint Strike Fighter F-35, a fifth-generation stealth jet, has been beset by spiraling costs and schedule delays. The program's price tag is nearly $400 billion for 2,457 planes -- almost twice the initial estimate.

To drive down costs and benefit from larger economies of scale, the Pentagon has sought to pool planned purchases through a "block buy" of hundreds of aircraft from the F-35's manufacturer, Lockheed Martin, according to the report by the Pentagon's Director of Operational Test and Evaluation, J. Michael Gilmore.

The report cautions that these efforts to curb costs have created a situation in which the U.S. could be committed "to procuring as many as 270 U.S. aircraft" before "operational testing is complete."

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