Northrop Grumman Corp. plans a 20 percent increase next year in the local production of its center fuselage sections for the F-35 Joint Strike Fighter as the program shifts to full scale production.

The stealthy, supersonic fighter, also called the F-35 Lightning II, is designed to replace several types of aging fighter and strike aircraft. Lockheed Martin Corp. is the lead contractor for the jet, which is being made in three versions for the U.S. Air Force, the Marine Corps, the Navy and allied defense forces around the world.

Northrop designed its portion of the project in El Segundo and assembles the center fuselage in Palmdale, using components made at its Los Angeles County plants and third-part contractors. The center fuselage is then shipped to Fort Worth for final assembly. Around 275 Northrop workers in Palmdale work on the project.

The company held a ceremony Tuesday to celebrate completed construction of the first center fuselage for the Navy version of the jet, signaling the company is starting to move from the system development and demonstration to production. The company so far has assembled completed 12 center fuselages for other versions of the aircraft.

Northrop has delivered seven of its 10 planned sections so this year, the most since shipments began in 2005. The company plans to pick up the pace to about one a month in 2009 for a total of 12, said F-35 program manager Mark Tucker in an interview with Bloomberg News.

The $298 billion F-35 program is considered the U.S. military’s most expensive program.