Lockheed Martin Corp is being awarded a $4 billion fixed-price U.S. Navy contract for 30 F-35 Joint Strike Fighter aircraft, the fifth batch of low-rate initial production, the Defense Department said on Friday.

The deal would provide 21 conventional models for the Air Force, six carrier variants for the Navy and three short-takeoff and vertical landing versions for the Marine Corps, a notice in the Pentagon's daily contract digest said.

Details of the deal have not been worked out, said Michael Rein, a company spokesman, referring to the government announcement as an undefinitized contract action subject to further negotiation.

This is welcome news for both Lockheed Martin and our many F-35 suppliers and will help ensure we continue to meet production schedules outlined by the program, he said by email.

The Pentagon currently plans to buy more than 2,440 F-35 aircraft in three separate models for the Navy, Marine Corps and Air Force.

It is the costliest Pentagon purchase in history, at a projected cost of $382.5 billion through 2035. The F-35 has been developed with eight foreign partners to replace at least 13 types of aircraft, including Lockheed's F-16, for 11 nations initially.

The Defense Department already has restructured the F-35 program twice in recent years. The fifth production batch fell to 30 from a previously planned 42 because more retrofits and changes have been required than had been projected.

The department and Lockheed will share modification costs tied to certain required fixes that may emerge during work on the fifth batch, the contract announcement said without spelling out the cost split.

The Pentagon's F-35 program director said in an interview published last week that the military should slow purchases of the F-35 because of the number of airframe cracks and hot spots turning up in testing and analysis.

Hot spots are potential structural flaws that reveal a durability or damage tolerance shortfall below the 8,000 flight hour or 30-year service life goal of the F-35, Kyra Hawn, a spokeswoman for the Pentagon F-35 program office, said in an e-mailed reply to Reuters.

(Reporting by Jim Wolf; editing by Carol Bishopric and Bob Burgdorfer)