The first flight of the MB 300 (affectionately known as “Big Julie”) took place on 15 or 16 November 1935, flown by André Curvale and Jean Lapeyre.
In 1935, in parallel with the twin-engine programme that would lead to the MB 220, a programme was issued for a fast 30-seat three-engine aircraft for Air France. Two projects were selected: the Dewoitine 620 and the MB 300.
Like the MB 200, the new model retained the wing of the MB 210, combined with a new fuselage and new tail unit. It was a three-engine low-wing monoplane, of all-metallic construction, with conventional retractable undercarriage and split flaps. The fuselage, which had a rectangular section with rounded top, offered four-abreast seating with a 55cm centre aisle. The 30-passenger cabin was divided into three sections: “first class”, with eight seats, a bar, then six seats; and “second class”, with 16 seats. The baggage holds were in the lower fuselage. The cockpit was designed for a crew of four: two pilots, radio operator and navigator. The aircraft was powered by three 1,000hp Gnome-Rhône 14 Kfrs 1 engines.
Baptized “Pacific”, but affectionately known as “Big Julie”, the MB 300 was taken to the Villacoublay airfield in July 1935. Ground runs got under way in October and the first flight took place on 15 or 16 November 1935, flown by André Curvale and Jean Lapeyre. The aircraft received the temporary registration F-AONB, pending receipt of the definitive airworthiness certificate.
On completion of initial testing, several modifications were introduced in early 1936, including a reduction in wing span and a 31cm fuselage stretch. The aircraft resumed flying on 2 February 1936 until July, when it was transferred to the CEMA test centre. At the end of November 1936, it arrived in Marignane for further testing. On 24 March 1937, it underwent another series of modifications: reduction in the number of seats from 30 to 24 (eight in the front, eight in the middle section with the bar, and eight in the rear), increase in tail area and installation of 14N16/17 engines. It returned to the CEMA and finally obtained its certificate of airworthiness with the definitive registration F-AOUI.
In early October 1937, the aircraft, leased to Air France by the state, started 100 hour endurance testing in the hands of André Curvale and Lionel de Marmier, chief flight test engineer at Air France. The aircraft finally officially joined the Air France fleet in January 1938. Its ultimate fate is unknown; reports say it was delivered to Spain.