The MB-326 was a low-wing monoplane with an all-metal (light alloy) structure. Powered by a Rolls-Royce Viper non-afterburning turbojet with low air-intakes in the wing roots. Each wing had 22 ribs and two spars. The fuel system had one large tank in the middle-fuselage and two in the wingtips. The aft fuselage was almost entirely dedicated to the engine, from just behind the wings. The cockpit had a tandem configuration, which was chosen to give a better aerodynamic fuselage (slimmer) than the more usual side-by-side arrangement. There was a long, low bubble canopy. The rear of each wing had flaps, and ailerons with a trim surface. Wing fences were added mid-wing to increase the lift characteristics.
In the 1950s, a number of countries were operating small jet trainers with a similar performance to their operational aircraft. At this time, several nations commenced development of purpose-built aircraft for the role, such as the Fouga Magister, the T-37, the Jet Provost, and the Aero L-29. Italy, which was still recovering from the effects of the Second World War, could not afford the development of supersonic interceptors or bombers; it instead elected to focus its development efforts on light fighter and trainer aircraft.
The MB-326 was designed by Ermanno Bazzocchi at Macchi. Bazzocchi considered many configurations before it was chosen to proceed with a single-engined design. The airframe was a robust and light structure, all-metal, simple and cheap; powered by an efficient engine, the Armstrong Siddeley Viper. This engine was designed as a short-life unit originally destined for target drones, but showed itself to be far more reliable. This airframe and engine combination led, in 1953, to the MB-326 project.
The Italian Air Force was quite interested, and so the MB-326 took part in the contest.
The contest specifications were:
- Max load 7 g at maximum weight
- 5,000 hours lifespan, 50–60 hours between servicing, stall-alert (at 15 km/h (9 mph) more than stall speed)
- Take-off at max load in 800 m (2,625 ft) over a 15 m (50 ft) high obstacle, or 500 m (1,640 ft) at light weight, landing in 450 m (1,480 ft) at minimum weight
- Speed (min-max): 110/130–700 km/h
- Rate-of-climb must be at least 15 m/s (2,950 ft/min) and endurance should be three hours at 3,000 m (9,840 ft).
There were several modifications to the MB-326 project: the horizontal tail surfaces lost their negative dihedral angle, the airbrakes (two in the wings) became one, in the ventral position. In 1956 the AMI approved the project and requested two prototypes (MM.571 and 572) and one airframe for static tests. No weaponry or pressurization was needed, but Bazzocchi introduced them.
The first prototype made its maiden flight on 10 December 1957, flown by Chief Test Pilot Guido Carestiato, and the second flew the following year. The plane showed very good characteristics, but the modifications affected the weight, which was 400 kg (880 lb) more than the initial estimates. The original Viper 8 engine produced 7.8 kN (1,750 lbf) of thrust, so the Viper 9 was adopted, which had 0.7 kN (147 lbf) more of thrust.
I-MAKI, the prototype, was first demonstrated in France. The second prototype first flew on 22 September 1958. It had a new Viper engine, the '11' model, updated to produce 11.1 kN thrust (1,134 kgf, 2,500 lbf).
On 15 December 1958, the AMI placed an order for 15 pre-series examples. In 1960, an order for 100 aircraft was placed, establishing Aermacchi's supremacy in jet trainers.
Direct competition came from the Fiat G.80, being more powerful and the first real Italian jet, having flown five years earlier, but it was also heavier, bigger and more expensive. It lost the contest, remaining without a market.