Upgrades available for the Silent Eagle include active electronically scanned array (AESA) radars, radar absorbent coatings, large digital cockpit displays, fly-by-wire software, canted tails and bolt-on internal weapons bays.
Still, Boeing must soothe Korean worries that the new jets, which the U.S. Air Force won't use, would cause logistics support problems if it is ordered only in small batches.
Sweden's Saab is also competing for the KF-X, offering a new design for a super-cruising stealth fighter, but its campaign has been marred by bribery allegations.
Saab wanted South Korea to join its next-generation Gripen program to forge a broader fighter development partnership. But the effort was marred after South Korean prosecutors arrested a former Air Force major general working with Saab for leaking classified military information to the company.
KF-X Partnership:
The KF-X program, which in 2007 was assessed as not viable economically and technically, has been postponed due to the small increase of defense budget.
But Seoul recently signed up its first international partner. On July 15, giving impetus to the controversial KF-X plan, Indonesia agreed to join South Korea's KF-X development effort and bear 20 percent of the initial budget of $8 billion to build an advanced fighter based on a Western aircraft platform. The Southeast Asian nation will buy 50 aircraft when mass production starts.
South Korea intends to foot 60 percent of the development costs, and is seeking to invite more foreign nations and companies to cover the remainder.
Potential partners include Turkey and the United Arab Emirates, as well as the F-X bidders Boeing, Lockheed Martin and EADS, according to the Defense Acquisition Program Administration (DAPA).
A joint team comprising experts from South Korea and Indonesia will build five prototypes of a KF-X aircraft before 2020, DAPA officials said.
After achieving the break-even point target of some 200 units, the aircraft would be ready for mass production.
The original proposal was to develop a fifth-generation stealth fighter, something that is in between the Dassault Rafale or Eurofighter Typhoon and the Lockheed Martin F-35.
Amid controversy over the feasibility of the original plan, the DAPA commissioned a second study in April last year to the Weapons Systems Concept Development and Application Research Center of Konkuk University.
The center then recommended a plan to develop an F-16+ aircraft fitted with the AESA radar, an electronic warfare suite and data link systems fit for a network-centric environment.
The think tank also recommended the jet have a combat radius about 1.5 times that of the F-16, an airframe life span 1.34 times longer than that of the F-16, better avionics than that of the F-16 Block 50.
Among other required capabilities are thrust of 50,000 pounds, provided by either one or two engines, super-velocity intercept and supercruise capabilities, and the ability to hit targets in the air, on land and at sea.

SEOUL - U.S. and European fighter jet makers are gearing up for South Korea's third-phase fighter acquisition competition set for next year.
The South Korean Air Force wants to replace its 200 McDonnell Douglas F-4Es and Northrop F-5 E/Fs - decades-old fighter jets that have seen several fatal crashes in recent years
- with a mix of state-of-the-art, imported and indigenous aircraft by 2020.
The service has also been upgrading its F-16s.
In 2000, South Korea launched the F-X program, an effort to buy 120 foreign fighters by 2020. Boeing won the first two stages, obtaining orders for 60 F-15Ks. The third stage, dubbed F-X III which encompasses 40 to 60 aircraft, will see requirements released next year and a final contract in 2012.
But Seoul is also pushing ahead with KF-X, an effort to work with foreign manufacturers to design and build at least 120 indigenous fighters on par with the F-16 Block 50. After several years of debates, the government decided earlier this year to move ahead and devote 4.4 billion won ($3.6 million) to exploratory development next year.
South Korea likely will link the F-X III to the KF-X in a bid to acquire advanced Western aircraft technology and increase its leverage in F-X negotiations.
Eurofighter, which has long vied for Korean aircraft contracts, sees the third phase as an opportunity to pierce the wall of U.S. dominance here. The European company says South Korea could receive lenient technology transfers, should the country join its Typhoon program, which currently has about 700 aircraft under contract with five European nations and Saudi Arabia.
An official responsible for the Eurofighter campaign in Seoul claimed the fighter allows options and upgrades that are unavailable for American jets.
"If you have an F-16 or F/A-18 fighter, for example, you're limited to how far you can upgrade it, given that they are both old platforms," the official said.
Lockheed Martin's F-35 is seen as a leading candidate for F-X III, whose requirements are expected to include a measure of stealth. But officials here have been rattled by the program's cost overruns, potential delivery delays, and the operational risks of adopting an early version of the plane.
Randy Howard, senior director at Lockheed Martin Aeronautics Co., dismissed the concerns.
"The F-35 is a centerpiece of security cooperation between the U.S. and key partners and allies, and a future sale to Korea is first and foremost about a stronger security relationship with the United States government," Giese wrote in an e-mail. "We believe that the F-35 - the world's only internationally available 5th-generation fighter - fully meets Korea's acquisition timelines and will provide unprecedented level of multi-role capability in both air-to-air and air-to-surface missions."
Giese said the price of an F-35 will depend on South Korea's requirements, but will be comparable to the fourth-generation fighters it would replace.
"Lockheed Martin fully supports Korea's fighter force recapitalization efforts through an open and transparent competition," he said.
Boeing, the reigning champion of the F-X competition, is aiming to win the third tranche with a stealthy variant of its twin-engine F-15. Within the month, the Chicago-based defense giant expects to receive an export license for its new F-15 Silent Eagle, which carries export-restricted low-observable technology.
On July 8, the Boeing Silent Eagle flight demonstrator aircraft F-15E1 completed a first flight from Lambert St. Louis International Airport, the company said in a news release.
"The Silent Eagle demonstration flight validated our initial engineering design approach," said Boeing F-15 Development Programs Director Brad Jones.
In the next couple of weeks, the F-15E1 will test-launch an AIM-120 missile, Jones said.
Gregory Laxton, vice president of Boeing Korea, attempted to defuse concerns about older systems by saying the Silent Eagle's gear is "state-of-the-art," even though its design is almost the same as existing F-15s.