Though sometimes described as a tourer[1] and at others, incorrectly, as a fighter aircraft,[2][3] the chief purpose of the Delanne 20-T was to explore the characteristics of the tandem wing Arsenal-Delanne 10, which was a fighter, larger than the 20-T and more powerful but aerodynamically very similar; the 20-T, the design of which was underway in the spring of 1937,[4] is best described as a macquette or aerodynamic model of the Arsenal-Delanne 10.

The Delanne 20-T was powered by a Regnier130 kW (180 hp) 6B-01 six cylinder air-cooled inverted inline engine, driving a two blade propeller.

[2] The Delanne 20-T had tailwheel, fixed landing gear, each of its mainwheels independent mounted on a pair of converging tubular legs to the lower fuselage.

On its second test flight on 10 August[Notes 1] it crashed after engine failure, killing pilot Guy de Chateaubrun.

[7] Testing and development flying, conducted by Fernand Lasne, was completed in June 1939 with satisfactory results,[8] after which attention concentrated on the Arsenal-Delanne 10.