The G.91Y was an increased-performance version of the Fiat G.91 funded by the Italian government. Based on the G.91T two-seat trainer variant, the single Bristol Orpheus turbojet engine of this aircraft was replaced by two after burning General Electric J85 turbojets which increased thrust by 60% over the single-engine variant. Structural modifications to reduce air frame weight increased performance further and an additional fuel tank occupying the space of the G.91T's rear seat provided extra range. Combat maneuverability was improved with the addition of automatic leading edge slats.

The avionics equipment of the G.91Y was considerably upgraded with many of the American, British and Canadian systems being licence-manufactured in Italy.

Flight testing of three pre-production aircraft was successful, with one aircraft reaching a maximum speed of Mach 0.98. Air frame buffeting was noted and subsequently rectified in production aircraft by raising the position of the tail plane slightly.