WASHINGTON - U.S. Defense Secretary Robert Gates will visit a Lockheed Martin Corp production plant for the F-35 fighter aircraft, the Pentagon's costliest arms purchase program, and an L-3 aircraft facility, highlighting their importance to the U.S. Defense Department, a Pentagon spokesman said on Thursday.

Gates, on his way back from vacation on Monday, will speak to reporters at the Lockheed plant in Fort Worth, Texas, and then pay a visit to an L-3 Communications Holdings Inc plant that turns out so-called Project Liberty reconnaissance aircraft in Greenville, Texas.

These are two very important aviation programs to the Department of Defense, said Bryan Whitman, a Pentagon spokesman.

The visit comes amid mounting speculation the Pentagon might have to scale back the F-35 as part of stepped-up budget belt-tightening.

For instance, the private Center for Strategic and Budgetary Assessments -- several of whose one-time experts are now serving in senior Obama administration jobs -- cited the F-35 as just one example of programs ripe for review by the Department of Defense during a once every four years top-to- bottom reassessment now under way.

Rather than buying both new long-range bombers and thousands of short-range F-35 fighters, DoD might consider whether the new bombers ... could represent a cost-effective substitute for some number of these new fighters, the center's Todd Harrison wrote in a report released this month.

Lockheed is developing three radar-evading F-35 models to replace at least 13 types of aircraft for 11 nations initially. At a projected cost of about $300 billion over two decades, the United States currently plans to buy 2,443 F-35s for the Air Force, Navy and Marine Corps.

Eight countries are F-35 co-development partners: Britain, Canada, Italy, Denmark, Netherlands, Norway, Turkey and Australia.

Northrop Grumman Corp and BAE Systems Plc are Lockheed's chief F-35 sub-contractors. Two separate, interchangeable F-35 engines are under development. One is built by United Technologies Corp's Pratt & Whitney unit. The other by a team of General Electric Co and Rolls-Royce Group Plc. (Reporting by Jim Wolf; editing by Andre Grenon)