Which production fighter was the first designed on purpose to be aerodynamically unstable?

The F-16 was the first production fighter deliberately designed to be slightly unstable aerodynamically, a characteristic known as relaxed static stability (RSS), adopted both to reduce drag and to improve manoeuvrability. The result is a highly agile fighter, the first built expressly to sustain 9 g manoeuvres and capable of exceeding Mach 2.

The conceptual starting point was energy-manoeuvrability theory, which Colonel John Boyd developed together with the mathematician Thomas Christie to model a fighter's combat performance, drawing on his experience in the Korean War and as a fighter tactics instructor in the early 1960s. Northrop's YF-17 prototypes first flew on 9 June and 21 August 1974. During the fly-off, the YF-16s completed 330 sorties and 417 flight hours, compared with 288 sorties and 345 hours for the YF-17s. The YF-16 moved from one manoeuvre to the next significantly faster and was the unanimous choice. Another advantage of the YF-16, unlike the YF-17, was the Pratt & Whitney F100 turbofan, the same one used by the F-15. The YF-17 was later developed into the YF-18 for the Navy.

The pilot sits at an unusual reclined angle of 30°. Instead of the traditional centre stick, the aircraft uses a side-stick controller mounted on the armrest.