In 2014, the C103 configuration was chosen and Lockheed Martin agreed to transfer two dozen F-35A technologies as part of a purchase deal. However, the US government blocked the transfer of four vital technologies: AESA radar, infrared search and track (IRST), electro-optical target tracking devices, and radio jammer technology. South Korea was thus required to develop these technologies domestically. A 2015 audit estimated that 87% of technologies for the project had been secured. The preliminary design was finalized in June 2018. In September 2019, a critical design review examined 390 technical data sets and confirmed that the KF-X was adequate to ROKAF's requirements.