Although resembling its predecessor, the Fiat G.91, the aircraft was in fact a complete redesign, a major difference being that it was equipped with a new twin-engine configuration, replacing the original single engine.

[2] Structural modifications to reduce airframe weight increased performance further and an additional fuel tank occupying the space of the G.91T's rear seat provided extra range.

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

Airframe buffeting was noted and subsequently rectified in production aircraft by raising the position of the tailplane slightly.

The last one was delivered around mid 1976, making a total of two prototypes, 20 pre-series and 45 series aircraft.