Lockheed is developing three models of the new fighter for the U.S. Air Force, Navy and Marine Corps, as well as eight countries that helped fund its development: Britain, Australia, Canada, Denmark, Turkey, Italy, the Netherlands and Norway.
After years of political wrangling, the Netherlands in September became the seventh foreign country to make a firm commitment to buy F-35s, joining Britain, Italy, Australia, Norway, Israel and Japan.
Britain is expected to announce additional orders next month, and Turkey is likely to become the eighth foreign buyer in January when it is expected to place firm orders for two of the 100 jets it plans to buy over time.
Japan and Israel are expected to order more jets next year, the sources said, while Singapore and Belgium are also considering joining the program.
U.S. government and industry officials also cite strong interest in the F-35 in the Gulf region, and say they have begun looking at when to release the jet to the region -- probably about five years after Israel gets its first jets in 2016.
Once Seoul - as new buyer - formally notifies the Pentagon about its planned purchases, those jets will be added to the total number of expected purchases by the U.S. military and allies that is used by defense officials to estimate the cost of each airplane.
By 2019, the Pentagon projects the cost of each new F-35 fighter plane will be around $85 million, putting it on a par with the cost of current fighter planes, said Jim McAleese, a Virginia-based defense consultant.
The South Korean order could create 10,000 jobs at Lockheed and its suppliers as they build the components to make the 40 jets.
The sources said the projected savings and job counts were comparable to similar estimates released when Japan announced plans to buy 42 F-35 jets from the U.S. government in December 2011