South Korea's decision to sacrifice force structure in favor of buying into stealth with the F-35—a boon for Lockheed Martin's premier fighter program—quashes Boeing's hopes of selling it a semi-stealthy F-15.

Seoul plans to buy 40 F-35As to satisfy its F-X Phase 3 fighter requirement, initially set at 60 aircraft, its joint chiefs of staff announced last month. Another 20 fighters, not necessarily F-35s, may be ordered later, subject to security and fiscal circumstances. The choice was influenced by overwhelming support in the South Korean military for the F-35. The buy was cut by one-third to keep it within an 8.3 billion won ($7.2 billion) budget.

Though it is a long shot, Boeing is still pushing the Silent Eagle for the remaining requirement of 20 fighters. Dennis Muilenburg, president of Boeing Defense, Space and Security, said in October that the country might be open to a mixed buy, and news of Seoul's F-35 decision has not changed that position.

“The Silent Eagle is still available,” Karen Fincutter, a Boeing spokeswoman, said last week, adding that the company had not yet been officially notified of South Korea's decision. “It's one of the upgrade options for the F-15.”

The other contender for South Korea's fighter program, the Eurofighter consortium, led by EADS in South Korea, also seems to have a remote chance of filling the later 20-aircraft order with the Typhoon.

Lockheed Martin is expected to deliver the aircraft over four years beginning in 2018. If the F-35 is selected, the other 20 would immediately follow.

The air force effectively chose the F-35 about two weeks ago when, according to local media reports, it set a numerical requirement for radar cross-section. This followed the derailment of the original competition in August, when it was revealed that only Boeing could meet the budget. Instead of completing the competition in the usual way by giving the order to a particular company, the government decided to review its requirements, budget and process.

Most of the South Korean air force leadership has always seen the F-35 as the desirable aircraft, according to government officials, though at the last minute the air force chief was willing to take F-15s so that new fighters could enter service without further delay.

F-X Phase 3 has been aimed at replacing old and increasingly ineffective F-4 Phantoms and F-5 Tigers. Under Phases 1 and 2 in the last decade, South Korea ordered 61 F-15s, including a replacement for one that crashed.

The selection in favor of high-end stealth is a major blow to Boeing's hopes of selling semi-stealthy Silent Eagles to its longtime F-15 customer and thereby keeping the production line active. Boeing is now left with the F/A-18E/F Super Hornet, modified with low-observable features, as its sole potential competitor to the F-35 for countries seeking near-term stealth fighter options.

“The F-35A will be used as a strategic weapon to gain a competitive edge and defeat the enemy in the early stage of war,” the Yonhap news agency quotes the joint chiefs of staff spokesman, Eom Hyo-sik, as saying. “The South Korean military will also use the aircraft to effectively deal with provocations.”

If the additional 20 F-35s are ordered, the requirements will be reconsidered, he adds, though industry officials expect no real competition.

In the same meeting last week, the joint chiefs endorsed the KF-X indigenous fighter program for the mid-term defense budget plan. That does not assure its survival, however. The fiscally and technically challenging KF-X faces stiff political opposition, and its 2014 funding is expected to cover only further design studies.

The military wants KF-X development to be wrapped up by 2020, with deployment starting in 2023, Yonhap says, citing unnamed officials. The three-year gap between the completion of development and beginning of deployment is not explained, but 2020 is not a realistic date by which the aircraft can be ready for service, anyway. The program schedule puts first flight seven years after the launch of full scale development, which cannot now happen until 2015.