Marcel Bloch's offering, the Bloch 130M, flew for the first time early in June 1934 at Villacoublay, the fixed undercarriage subsequently being replaced by an undercarriage of retractable type. After some refinement of the aircraft, an order was placed in October 1935 for 40 Bloch 130Ms. However, both the Air Staff and Avions Marcel Bloch had, by this time, begun to appreciate the fact that the Multiplace de Combat concept was no longer valid, and Bloch had already initiated the design of an appreciably more advanced and rather more specialized aircraft, the Bloch 131 RB4, a four-seat bomber-reconnaissance aircraft that had nothing in common with the Bloch 130M. Pending the outcome of prototype trials with the new design, the order for the earlier aircraft was accordingly shelved.
The Bloch 131 No. 1 was built at Courbevoie and assembled at Villacoublay where the first flight took place on August 12, 1936. A second prototype, the Bloch 131 RB5 No. 2, was exhibited at the Salon de l'Aéronautique (Paris, November 1936), this differing from the initial machine in accomodating a crew of five, the dorsal turret had been moved further aft, a ventral turret had been introduced, the chord of the fin had been increased, and the bracing struts had been removed for the tail plane.
This prototype was flown for the first time on May 5, 1937, and in the meantime, the first prototype had been flown from Villacoublay to Marignane for CEMA (Centre d'Essais du Matériel Aérien) trials on December 6, 1936. Further CEMA trials had been conducted at Villacoublay in April, and the earlier order for 40 Bloch 130Ms was supplanted by an order for 40 Bloch 131s. Both prototypes were powered by 900 hp Gnome-Rhone 14K Mistral-Major fourteen-cylinder air-cooled radial engines, but in the summer of 1937, by which time Avions Marcel Bloch had been absorbed by the Société Nationale de Constructions Aéronautiques du Sud-Ouest (SNCASO), these powerplants were replaced by the 950 hp Gnome-Rhone 14N 10/11 engines, also used in the production aircraft.
The first prototype was experimentally fitted with a 0.787 in (20 mm) cannon on a flexible mount in the extreme nose, however, this weapon had been abandoned in favor of a 0.3 in (7.5 mm) MAC 1934 machine gun when, in September 1937, the second prototype underwent firing trials at Cazaux. The first production aircraft, the Bloch 131 No. 3, was fitted with a semi-retractable ventral gun position, but soon it was replaced by a shallow fixed semi-glazed gondola containing a single MAC 1934 machine gun.
The first production machine from the SNCASO's Châteauroux-Déols plant was flown in May 1938, the initial batch of 14 aircraft (Nos. 3 to 15) being Bloch 131 R4 four-seaters equipped solely for reconnaissance tasks, these being followed by five (Nos. 16 to 20) Bloch 131 Ins dual-control instructional aircraft. The second prototype was also modified to Bloch 131 Ins standards and delivered to the Armée de l' Air. The nineteenth production machine (No. 21) was the first Bloch 131 RB4 intended for the dual bombing-reconnaissance role, and approximately 100 aircraft of this model were subsequently manufactured by SNCASO's Chateauroux-Déols and Bordeaux-Mérignac plants.
Externally, the Bloch 131 RB4 displayed no differences from the R4, but the main wheels were enlarged, and the internal bomb-bay could be fitted with various types of Alkan racks for bombs ranging from 22 to 441 lb (10 to 200 kg) in weight. The navigator-bombardier in the extreme nose, who also operated the Labrely F30 or F50 camera, was provided with a single MAC 1934 gun, and similar weapons were mounted in the dorsal turret and in the ventral gondola, that in the latter position being fired by the radio-operator.
Only one experimental development of the basic Bloch 131 was tested, this being the Bloch 133 which, flown on October 1, 1937, retained the Bloch 131 fuselage with the standard tail assembly replaced by oval endplate fins and rudders, and Hispano-Suiza 14AA radials in place of the Gnome-Rhones.
Span: 66 ft 6 in (20.27 m)
Length: 58 ft 6.75 in (17.85 m)
Height: 13 ft 5 in (4.09 m)
Wing area: 581.25 sq.ft (54 sq.m)
Weight empty: 10,340 lb (4,690 kg)
Loaded weight: 17,438 lb (7,910 kg)
Max speed: 217 mph (350 kmh) at 12,300 ft (3,750 m)
Cruise speed: 168 mph (270 kmh) at 12,300 ft (3,750 m)
Service ceiling: 23,786 ft (7,250 m)
Climb: to 13,123 ft (4,000 m) 13 min
Range: 808 mls (1,300 km)."