A total of ten flying and three non-flying prototypes were built for flight tests and initial combat trials. Five flying and two non-flying prototypes comprise the "first stage" aircraft design, with the two non-flying prototypes testing static flight loads and avionics integration. The first two flying prototypes tested flight characteristics, while the second two prototypes conducted airborne tests of avionics systems, including the radar and electronic warfare suite. The second prototype has since been used to test the second stage "Izdeliye" 30 engine. The fifth prototype was severely damaged by an in-flight fire, and the remains were combined with parts cannibalized from the sixth prototype in order to return the aircraft to flight status. Starting with the sixth flying aircraft, five more of the structurally redesigned "second stage" aircraft were built, as well as one non-flying prototype to test flight loads on the new structure. The last two prototypes were test articles of production aircraft. Issues and accidents during the testing resulted in repeated delays to the programme, with the first production aircraft originally planned for delivery in 2015; this has been delayed multiple times, and the date is now expected to be in 2020.