be delivered to the French Air Force by 1 April 1939; this was later cut down to 206 aircraft. In reality, only a single aircraft had been delivered by the prescribed deadline; other aircraft types also proved similarly unable to attain the tight delivery dates. Further development However, there was no direct production of the MB-150.01 as the aircraft having been deemed to be unsuitable for mass production. Amongst other changes needed, the structure of the airframe had to be redesigned in order to suit mass production. During early April 1938, an order was received for a three further prototypes; these were to explore the possibilities for installing more powerful engines of both French and American origins, such as the Hispano-Suiza 14AA, Pratt & Whitney R-1830 Twin Wasp, and further derivatives of the Gnome-Rhône 14N engine. Accordingly, this design effort led to the production of the MB.151.01 and MB.152.01 prototypes, which were developed and produced in parallel. The first pre-production prototype, the MB.151.01, was quickly assembled at Courbevoie using the new simplified construction methods developed. This aircraft, which was fully armed, performed its first flight at Villacoublay Airfield, Île-de-France, on 18 August 1938. According to Christesco, the performance of the MB.151.01 was initially disappointing, leading to efforts to rectify performance issues. Development, and thus mass production, was delayed by the overheating of the engine (resulting in oil cooler types being tested and the most efficient of these adopted) and the aircraft being poorly balanced on its pitch axis at high speeds; neither the prototype nor the production MB.151 were able to attain 480 km/h, the design's estimated maximum speed. According to Christesco, the MB.152.01 was "the first true aircraft" of the series. This model was equipped with a more powerful 1,030 hp Gnome-Rhône 14N-21 engine, capable of a speed of 520 km/h and equipped with a revised armament arrangement. On 15 December 1938, the MB.152.01 prototype performed its maiden flight. During January 1939, it was refitted with a more production-representative 1,000 hp Gnome-Rhône 14N-25 engine; various alternative engine cowlings and propellers were also tested to address engine overheating. To prevent further delays to the production aircraft, a large cowling was adopted, which increased drag and reduced the MB.152's flight performance. The manufacturing of the fighter was divided amongst the various branches comprising SNCASO. Aside from a handful that were assembled at Courbevoie early on, roughly half of all aircraft produced