The KAI KF-21 Boramae (; formerly KF-X; commonly KF-21) is a South Korean twin-engine fighter aircraft. The initial goal of the program was to develop an indigenous multirole fighter for the Republic of Korea Air Force (ROKAF). The airframe of Blocks I and II adopts semi-stealth technology with partially embedded external weapons and MUM-T (Manned-Unmanned Teaming) capabilities, while full stealth, including internal weapons bays is planned for the Block III or KF-21EX variant.T. Washington Institute As South Korea's second domestic fighter jet development program following the FA-50 series, the KF-21 makes the nation the fourth in the world—joining the United States, China, and Russia—to produce an aircraft featuring stealth avionic architecture.
The Block I program is led by the South Korean government, which holds 60% of the shares. An additional 20% is held by the manufacturer Korea Aerospace Industries (KAI), with Indonesia holding the final 20% stake. Later, in August 2024, Indonesia's stake was reduced to 7.5% due to a request from the Indonesian government. The Block II program, which focuses primarily on the serial integration of air-to-ground and air-to-ship armaments, is already underway. This collaborative approach will extend into the Block III (KF-21EX) program, which is also set to be carried out with newly formed global partnerships.
In April 2021, the first prototype was completed and unveiled during a rollout ceremony at the headquarters of KAI at Sacheon Airport. It was named the Boramae after a juvenile Eurasian goshawk. The maiden flight was on 19 July 2022. Serial production started in July 2024, with the initial rollout taking place in March 2026. 40 Block I units are scheduled for deployment beginning in September 2026; the ROKAF expects to complete additional 80 Block II units' deployment by 2032. It is available for export. The ROKAF will initially begin replacing its aging F-4D/E Phantom II and F-5E/F Tiger II jets with the KF-21. Over time, the KF-21 will also replace its fleet of KF-16s and F-15Ks in service.
The first mass-produced KF-21 entered service with the Republic of Korea Air Force's 156th Fighter Squadron on 22 September 2026.
Background Announced in March 2001 by South Korean President Kim Dae-jung at a graduation ceremony of the Korea Air Force Academy, the KF-X project aimed to produce an advanced multirole jet fighter to replace South Korea's aging F-4D/E Phantom II and F-5E/F Tiger II aircraft. In 2002, research and development (R&D) requirements were determined by the Joint Chiefs of Staff. The project was considered extremely ambitious; the Korea Institute for Defense Analyses (KIDA), a state-owned defense ministry think tank, expressed serious doubt about the country's ability to complete such a complex undertaking. The ROK Air Force expressed concerns that a delay or failure of the KF-X program could significantly compromise its future security readiness. During this period, the project faced heavy scrutiny from local media, which maintained a predominantly pessimistic tone regarding its massive budget and technical viability.
Consequently, the program was subjected to a grueling cycle of ten separate feasibility studies, leaving the evaluation phase fraught with delays while its economic cost and potential risks were debated. However, the project received renewed interest after a feasibility study in 2008 and attacks by North Korea in 2010. The last feasibility study concluded that developing the domestic defense industry was considered a matter of national importance and expected to generate positive ripple effects across high-tech sectors despite the project's risks and the significantly higher per-unit cost compared to foreign alternatives. The study still required a foreign partner to mitigate financial risks.
To fulfil the foreign partnership requirement, South Korea also offered Turkey a 20% stake in the KF-X project after offering the program in 2009 to Indonesia that is accepted; however, Turkey declined, seeking greater control over the initiative. The South Korean government expected to commit 60% of the development cost, with the remaining 40% to be covered by domestic and foreign partners. As an alternative, Indonesia agreed to fund the 20% share through Indonesian Aerospace and purchase 50 of the approximately 150–200 planned aircraft. On 15 July 2010, South Korea and Indonesia formalised a partnership, launching the KF-X development program with a target to begin aircraft deliveries in 2026.
Design and development The initial goal for the program was to develop a single-seat twin-engine multirole fighter with stealth capabilities exceeding the Dassault Rafale, Eurofighter Typhoon, F-16 Fighting Falcon, but less than those of the Lockheed Martin F-35 Lightning II. The Weapon Systems Concept Development and Application Research Center of Konkuk University advised that the KF-X should be superior to the F-16 Fighting Falcon, with 50% greater combat range, 34% longer airframe lifespan, better avionics, active electronically scanned array (AESA) radar, more-effective electronic warfare, and data link capabilities. Their recommendations specified approximately of thrust from two engines, supersonic interception and cruising capabilities, and multi-role capabilities. The Block I configuration requirements were later finalised by the ROKAF and DAPA to a 4.5 generation fighter with semi-stealth capabilities.
Configuration development and selection
At the onset of configuration development, South Korea had achieved a 65% indigenous technology localization rate for the KF-X program, prompting efforts to both develop the remaining capabilities domestically and seek international partnerships to fill the gaps. The Agency for Defense Development (ADD) proposed two primary KF-X concepts: C103, resembling the F-35, and C203, modelled after European fighters with forward canards. These configurations were outlined for potential partnership with the United States or European partners.
On the other hand, Korea Aerospace Industries (KAI) and the Defense Acquisition Program Administration (DAPA) proposed the C501 ( KFX-E), a third design that prioritised cost reduction through a smaller, single-engine configuration. While more affordable, the C501 offered lower performance compared to the F-16 and was considered unsuitable for operations over Indonesia's large airspace.
Besides, ROKAF favoured a twin-engine design for its superior combat performance, enhanced survivability, and larger airframe capable of accommodating future upgrades. These enhancements could eventually allow the aircraft to be reclassified as a fifth-generation fighter, whereas the C501 remained closer to a fourth-generation standard. By December 2015, the development team had begun designing the KF-X, with initial research and design based on the C103 configuration. After receiving the basic drawings of the C103, C104, and C105 from ADD, the team built an experimental model of the C105 and conducted wind tunnel tests, which were then repeated on the newly designed C107 featuring a larger airframe and greater maximum takeoff weight. An audit in 2015 estimated that 87% of the technologies required for the KF-X program had already been secured by this time.
For the remaining technologies, Lockheed Martin initially agreed with South Korea on transfer of 25 technologies to KAI under an offset trade agreement linked to a contract to acquire 40 F-35As. However, the U.S. later disapproved the transfer of four core technologies - AESA radar, infrared search and track (IRST), electro-optical targeting pod (EO TGP), and radio frequency jammer (RF jammer) technology—requiring South Korea to develop these capabilities domestically. For the rest of 21 general technologies, the U.S. government approved the transfer to South Korea in December 2015, including avionics operational flight program (OFP) software, head-mounted displays (HMD), and aerial refueling control units. Despite its previous partnership on the T-50 design, Lockheed Martin's role in the KF-21 Boramae program was limited. While the company provided guidance to KAI's engineers during the KF-21's development and testing, Lockheed Martin does not possess any design rights to the fighter jet. It later emerged that Lockheed Martin had maintained a skeptical outlook, predicting that the KF-X program would eventually be cancelled or fail to reach completion.
For the initial armaments, the U.S. government delayed integration approval for the Raytheon AIM-120 AMRAAM and AIM-9X Sidewinder, South Korea moved decisively to secure alternatives, quickly integrating the European MBDA Meteor and Diehl IRIS-T to maintain the KF-21's development timeline. These European systems were selected to diversify the platform's strike options and arm initial production blocks while South Korea fast-tracks the development and integration of its own indigenous air-to-air missiles; notably, the U.S. later granted approval for its weapons systems only after the European integration had been finalised.
Following approximately two years of wind tunnel experiments and continued domestic technology development, the C109, jointly proposed by ADD and KAI, was selected in 2018 as the prototype design for the KF-X. The preliminary design was finalised in June 2018, and a critical design review in September 2019, examining 390 technical data sets, confirmed that the KF-X met ROKAF's requirements. The specific mandate for high manoeuvrability, low radar cross-section (RCS), and the higher survivability of a twin-engine configuration naturally dictated an airframe shape similar to the F-22 Raptor, as both aircraft were engineered to meet similar stealth-physics and performance benchmarks. However, in terms of physical dimensions, the aircraft's footprint is more comparable to the F-35 Lightning II than the larger Raptor.
+KF-X Design Configuration Comparison Alt URL KAIC501 ADDC103 ADDC104 ADD / KAIC105 ADD / KAIC107 ADD / KAIC109 Empty weight 9,300 kg(20,500 lb) 10,900 kg(24,000 lb) 11,090 kg(24,450 lb) 11,100 kg(24,420 lb) 11,600 kg(25,500 lb) 11,800 kg(26,000 lb) Max takeoff weight 20,900 kg(46,000 lb) 24,000 kg(53,000 lb) 24,100 kg(53,200 lb) 24,500 kg(53,900 lb) 25,800 kg(56,900 lb) 25,400 kg(56,000 lb) Internal fuel 3,600 kg(8,000 lb) 5,400 kg(12,000 lb) 5,400 kg(12,000 lb) 5,400 kg(12,000 lb) >5,400 kg(12,000 lb) 5,400 kg(12,000 lb) Wingspan 9.8 m(32 ft) 10.7 m(35.2 ft) 10.7 m(35.2 ft) 11.0 m(36.08 ft) 11.2 m(36.7 ft) 11.2 m(36.75 ft) Length 15.2 m(50 ft) 15.7 m(51.3 ft) 15.7 m(51.3 ft) 16.0 m(52.49 ft) 16.8 m(55.1 ft) 16.9 m(55.4 ft) Wing area 37.1 sq m(399 sq ft) 42.7 sq m(460 sq ft) 42.7 sq m(460 sq ft) 42.7 sq m(460 sq ft) 46.5 sq m(501 sq ft) Engine 1 × P&W F100or GE F110 2 × EJ200or GE F414 2 × GE F414 2 × GE F414 2 × GE F414 2 × GE F414 Maximum speed Mach 1.89 Mach 1.97 Mach 1.97 Hardpoints 9 10 10 10 10 10 Weapons bay None Space secured Space secured Space secured Space secured Space secured
Modular design and evolutionary approach
The KF-21 program's core philosophy centers on a spiral development model, where the aircraft is introduced in a series of progressively more capable versions, or blocks. This modular approach allows for incremental improvements, with each block introducing new capabilities and technologies. The initial focus was on developing a highly capable 4.5-generation fighter jet with scalability to be upgraded to a true 5th-generation aircraft in later iterations. It was intended to both fulfil the ROKAF's urgent need to retire its F-4 Phantom II and F-5 Tiger II jets and to secure opportunities for future upgrades.