The idea of a Korean-designed fighter was announced by President Kim Dae-jung in 2001, but the KF-X project spent years in feasibility studies and cost debate before KAI signed the full-scale development contract in 2015. South Korea funds the large majority of the programme; Indonesia signed on in 2016 as a risk-sharing partner, and US firms and Lockheed Martin provided some technology transfer, though several sensitive technologies (including key stealth and AESA elements) were withheld and had to be developed at home.

The result is often described as a stepping stone between the FA-50 light fighter KAI already built and a possible future fifth-generation aircraft. It is important to hedge the claims made for it: many performance and cost figures come from the manufacturer or the Korean government and have not been independently verified in combat or long-term service, because the type is only now entering production.

When the KF-X programme began, the United States declined to transfer four key technologies — among them the AESA radar, infrared search-and-track and electronic-warfare suites. Rather than abandon the project, South Korea funded domestic development, with Hanwha Systems and the Agency for Defense Development producing a national AESA radar that was fitted to the prototypes and tested in flight from the mid-2020s. Independent verification of its detection range and reliability is limited, and figures cited for it should be read as manufacturer or ministry claims until the type has served operationally.

The KF-21 is deliberately positioned as a 4.5-generation fighter. It borrows stealth-influenced shaping — canted tails, a faceted forward fuselage and internal routing — but the initial production aircraft carry missiles and bombs externally, which sharply increases radar cross-section compared with a true fifth-generation type such as the F-35. KAI has spoken of a later variant with an internal weapons bay and reduced signature, sometimes labelled KF-21EX, but that aircraft does not yet exist. Any description of the KF-21 as a “stealth fighter” should be qualified: it is semi-stealthy by design intent, with full low-observability reserved for a future step.

Development of the KF-21 has been budgeted at roughly 8.6 trillion won (on the order of US$6–7 billion) for research and development, with total programme spending including procurement reported near 18 trillion won over its life. Per-aircraft flyaway cost has been estimated at around US$80–110 million, though published figures have drifted upward and vary with exchange rates and block standard — treat any single number as an estimate.

No reliable cost-per-flight-hour figure exists in open sources yet, which is unsurprising for an aircraft that has only just finished its development flight-test programme. Operating-cost claims for the type are, for now, projections rather than measured data.