Lockheed Martin [LMT] has completed assembly of a fourth F-35B short takeoff/vertical landing (STOVL) for the Marine Corps.

The company now has one F-35 Lightning II aircraft in structural testing, two in flight test, six in final assembly and another 14 in various stages of production, the company said.

The Marine Corps is expected to operate about 340 F-35Bs. The United Kingdom’s Royal Air Force and Royal Navy, and the Italian Air Force and Navy also will operate the STOVL variant, which will be the world’s first STOVL aircraft to combine stealth with supersonic speed.

“The completion of our fourth F-35–and the growing line of aircraft now forming behind it–shows an emerging rhythm in our production line,” Dan Crowley, Lockheed Martin executive vice president and F-35 program general manager, said. “In just a few days we will have all three Lightning II variants in final assembly when we take delivery of the first F-35C carrier variant center fuselage. From the very first F-35, assembly quality has been unprecedented, and each successive aircraft is measurably better than the one that preceded it.”

The new aircraft was moved immediately to the flight line, where it will undergo an extensive battery of ground tests before its first flight in early 2009. The first F-35B made its inaugural flight June 11 and has completed nine missions. The first F-35A, a conventional takeoff and landing (CTOL) variant, has flown 45 times.