WASHINGTON (Reuters) - Lockheed Martin Corp (>> Lockheed Martin Corporation) is developing and building the F-35 Joint Strike Fighter, the Pentagon's most expensive weapons program, for three U.S. military branches and eight international partners.

The Pentagon released new cost projections for 78 major weapons programs on Thursday, including the F-35 program which showed the first decline in cost after years of increases and restructurings.

The partners who are helping fund the F-35's development include Britain, Australia, Canada, Turkey, Italy, Norway, Denmark and the Netherlands.

Israel and Japan have also placed orders and Singapore may soon follow suit, according to U.S. government sources. Lockheed is also bidding for a 60-fighter order from South Korea.

Following are some key figures about the F-35 program:

COST OF THE OVERALL PROGRAM

* A new U.S. Defense Department estimate put the cost of developing, testing and building the F-35 multi-role fighter jet at $391.2 billion, down from last year's estimate of $395.7 billion. The forecast assumes U.S. purchases of 2,443 production jets, on top of 14 test planes.

* Retrofits of existing planes to address problems found in flight testing are expected to total $1.7 billion over the first 10 production batches, according to the most recent report by the congressional Government Accountability Office (GAO).

* The cost of operating and sustaining the new planes is estimated to reach $1.1 trillion, assuming that they will be used for 50 years, according to the most recent Pentagon data, although those estimates are likely to be updated this fall.

* Senior Pentagon officials have said they consider that price tag unaffordable. Program officials are taking steps to reduce the projected operating cost, including hiring engine maker Pratt & Whitney, a unit of United Technologies Corp (>> United Technologies Corporation), to cut the fuel burn of the engine by 5 percent.

COST PER JET

* New cost estimates prepared by the Pentagon showed a drop in the projected average cost of the Air Force and Marine Corps variants over the life of the program, but the projected cost of the U.S. Navy variant for aircraft carriers edged higher.

* The Pentagon now forecasts that the conventional takeoff A-model will average a cost of $76.8 million per plane, excluding R&D costs, down which is $1.9 million less than the estimate provided last year. The B-model, which can take off and land like a helicopter, is slated to cost $103.6 million per plane.

* The latest estimate put the cost of the carrier variant at $88.7 million per plane, up $1.7 million from last year.

* The fifth and most recent batch of jets ordered by the Pentagon cost 4 percent less than the previous order, and prices should come down steadily in the future, according to Air Force Lieutenant General Christopher Bogdan, who runs the F-35 program for the Pentagon.

* Lockheed submitted a bid for the next two batches of jets in January and both sides hope to reach agreement this summer.

* The 22 Air Force models included in the last contracts cost $119 million each, according to Bogdan, compared to a price of $127 million per plane a year earlier.

* The cost of the three Marine Corps' B-models in the fifth order, which have a more complex engine to allow it to land like a helicopter, is estimated at $153 million per plane, down from $164 million a year ago, when the Pentagon bought 17 B-models, according to defense officials familiar with the estimates.

* The seven Navy carrier variants or C-models in the fifth batch cost around $139 million, down from $148 million a year earlier, according to estimates by U.S. defense officials.

Over time, as production quantities increase, the jets are expected to start dropping in price. The per-plane forecasts factor in foreign orders, which are not included in the U.S. development, procurement and operating cost.

Bogdan recently said he expected to reach the target price at least for the A-model by 2020, when Australia is due to start buying the first of the 100 F-35s currently in its plans.

Lockheed executives say they believe the government's estimates are too conservative, and predict that the price of the new warplane will be even lower once the company starts full-rate production later this decade.

Critics of the program say Pentagon cost projections are probably too low, noting that further technical issues may well arise during flight testing of the new fighter jet.

The jet is built by Lockheed at its Fort Worth, Texas, plant, with Northrop Grumman Corp (>> Northrop Grumman Corporation) and BAE Systems Plc (>> BAE Systems plc) serving as key suppliers. Engines are built by Pratt & Whitney.

(Reporting by Andrea Shalal-Esa; Editing by Bob Burgdorfer)