An F-35B Lightning II lifts off the flight deck of the amphibious assault ship USS Wasp on Oct. 24, 2011.

South Korea has formally decided to purchase 40 F-35 stealth fighter jets for $6.7 billion, South Korea and Lockheed Martin officials announced Monday.

The commitment by South Korea, which is boosting its air defenses amid ongoing tensions in the Pacific region, marks a "big day" for the F-35 program in Fort Worth, said Steve O'Bryan, Lockheed's vice president for F-35 business development.

"This is a two-year process that Korea held, an open competition where they looked at capabilities, their defense needs, the price as well as the timing and they chose the F-35. It was another open, transparent process that really verified the capabilities of the F-35 and what we are doing in Fort Worth," O'Bryan said.

"All of these jets will be built and delivered off the line here at our facilities. Our suppliers, a lot of them in Texas and Dallas-Fort Worth, will build all those parts," he said.

The F-35 currently costs $100 million per plane but the price is expected to drop to about $85 million in 2019, or about $75 million in today's dollars, he said.

The purchase will allow Lockheed Martin to ramp up production on the more than one-mile-long F-35 assembly line. About 2,000 people currently work on the line at the company's sprawling plant in west Fort Worth.

South Korea is the 10th country to make a firm commitment to buy the new Lockheed fighter, joining the United States, Britain, Australia, Norway, Italy, the Netherlands, Japan, Israel and Turkey.

Canada and Denmark, which help fund development of the F-35, are still deciding whether to buy F-35s or other fighters. Singapore has also expressed interest in the planes, O'Bryan said.

The South Korea announcement provided some good news for Lockheed after a series of critical reports on challenges with software development for the fighter, the emergence of additional bulkhead cracks during long-term durability testing, and news that Italy could further scale back its plans to buy 90 F-35s.

Italy had already cut its planned order by 30 percent two years ago.

The F-35 program includes participation from three U.S. military service branches and eight international partner countries. Three variants of the F-35 will replace the A-10 and F-16 for the U.S. Air Force, the F/A-18 for the U.S. Navy, the F/A-18 and AV-8B Harrier for the U.S. Marine Corps, and a variety of fighter aircraft for at least 11 other countries.

South Korea was the eighth-largest importer of major weapons in the world between 2009 and 2013, with 80 percent of the imports supplied from the United States, according to think tank Stockholm International Peace Research Institute.

The purchase is a tribute to Lockheed's 12,500-person workforce in Fort Worth, O'Bryan said.

"This is independent verification and another statement of what they have done in designing, sustaining and producing a fifth-generation airplane. Another of the world's great air forces has again selected the F-35," O'Bryan said.