The half-scale model pre-decessors of the SO 4000 were the SO M.1 glider (F-WFDJ) and the powered SO M.2 (F-WFDK), these featuring a laminar flow wing swept 31 deg at the mainspar, mounted in the high-mid position and combining small ailerons interconnected with spoilers and leading-edge slats. The undercarriage consisted of a retractable central skid with wingtip stabilising skids in the case of the M.1, while that of the M.2 comprised a nosewheel, three main-wheels in train and wingtip outrigger wheels.
The powered M.2 had entered flight test five months earlier than the M.1, on 13 April 1949, when it was flown for the first time at Orléans-Bricy with Jacques Guignard at the controls. The M.2 was powered but was in most other respects, similar to the M. 1.
During its seventh flight in May 1950, with Daniel Rastel at the controls, the M.2 exceeded 621 mph (1 000 km/h) in a shallow dive. For the second phase of its flight test programme, commencing in October 1951, the M.2 was fitted with a modified control system, a revised undercarriage retraction system, provision for the installation of powder rockets to augment thrust at high altitude and wingtip fuel tanks. Empty and loaded weights being raised to 9,369 lb (4 250 kg) and 11,905 lb (5400kg).
Engine: Rolls-Royce Derwent V turbojet, 3,000 lb st (1360 kgp)
Span: 3l ft 2 in (9,50m)
Length of 32 ft 5.75 in (9,90 m)
Wing area: 193.76 sq ft (18,00 sq.m)
Empty wt: 8,466 lb (3 840 kg)
Loaded wt: 10,362 lb (4 700 kg)