Pratt & Whitney and the U.S. Dept. of Defense briefly confirmed news of a new agreement in principle for the jet engine builder to produce the sixth series of F135 propulsion systems for the F-35 II Lightning joint-strike fighter program. According to separate reports, the contract is valued at more than $1 billion.

The final terms of the supply agreement have not been finalized. The order for the sixth round includes 39 engines, 36 for F-35 planes plus three spares.

In May, Pratt & Whitney finalized the terms of its estimated $1-billion contract to build 35 engines for the fifth series of the F-35 fighter program. The terms had been left unsettled when the series was approved for construction in February.

The F-35 Lightning II is a single-engine jet with stealth capability, to be used by the U.S. Air Force, U.S. Navy, U.S. Marine Corps, and Great Britain’s Royal Air Force, for ground attack, reconnaissance, and air defense missions. Lockheed Martin is the primary contractor for the jet building program, and it previously settled its terms for the fifth round of F-35 construction.

Pratt & Whitney has said the cost of the F135 engine it builds for the F-35 fighters is down about 40% since the program began in 2001, when the program began.

In the latest announcement, the U.S. Department of Defense and Pratt & Whitney jointly stated that production costs for the F135 program continue to decline.