SAB AB - 20 The SAB AB - 20 was a large four engine French bomber built in the early 1930s as a development of the Dyle et Bacalan DB - 70 airliner . It featured a lifting body of thick airfoil section between the inner engines . It was later modified for the attack role ; a second aircraft , the AB - 21 , had different engines and cleaner aerodynamics but no more were built . Design . The SAB AB - 20 was a four engine night bomber development of the three engine Dyle et Bacalan DB - 70 airliner . The change of manufacturer ' s name was the result of the financial failure of Dyle et Bacalan in 1929 , followed by its immediate reappearance as SAB , who took over DB - 70 development . The latter was built around a thick , wide chord airfoil centre section which provided generous internal space for passengers . The engines were mounted on this structure as were twin fuselages to carry the tail . Outer wings of normal thickness and chord , the cockpit and the under carriage were also attached to the centre section . The generous intra - wing volume equally offered crew , fuel and bomb room for military purposes . Initially the AB - 20 was intended to have three engines like its forebear , but during the design phase there was a military request for a bombardier ' s position and a gunner ' s cockpit in the nose , which required the removal of the centre engine and its replacement by two extra engines wing mounted outboard of the centre section . Apart from the extra engines and the very different crew compartment , together with the removal of passenger accommodation , the AB - 20 and DB - 70 had much in common : the thick centre section and high mounted outer wings , twin fuselages carrying a long horizontal stabilizer and twin fins and rudders . Both aircraft had conventional undercarriages with pairs of mainwheels widely separated on V - struts attached to the lower longerons of the centre section . The new central crew pod was flat sided and tapered forwards to a complicated cylindrical nose , formed of a simple lower part with an overhanging , windowed cabin for the navigator / bombardier and an open gunner ' s cockpit , fitted with a machine gun ring , directly above . The nose also carried a long , conical probe with fine extensions , possibly pressure sensors . Further aft there was an enclosed pilot ' s cabin . A second gunner was stationed rearwards , on top of the centre section and a third fired from a ventral turret . Development . The first flight of the AB - 20 was on 15 January 1932 . The following year it was reported that it had been structurally modified for the attack role and was returning to Villacoublay for tests . By early 1934 a much developed version , the AB - 21 had appeared . It had the newer V - 12 Lorraine Petrel water - cooled engines and a tapered , filleted cantilever wing without the struts used on the AB - 20 and DB - 70 . The undercarriage had also been cleaned up with streamlined legs and wheels in long cowlings . The nose was further complicated , retaining the upper , open gunner ' s position but now with double underhanging windowed positions , though the conical probe had gone . Trials of an airborne sideways - firing 75 mm gun were carried out with the AB - 20 prototype modified as the AB - 22 . Firing trials were halted after damage was caused to the lower wing skin by blast from the gun muzzle .