Editing Dewoitine D.520 (section)

===Modifications and first prototypes===
On 2 October 1938, the first prototype aircraft, D.520-01, powered by a [[Hispano-Suiza 12Y]]-21 engine that temporarily drove a fixed-pitch, two-bladed wooden propeller, performed its [[maiden flight]].<ref name="Danel 4"/> Completion of the prototype had been delayed somewhat by the need to incorporate modifications requested by the STAe following their examination of a wooden mock-up of the type. During early flight tests, the first prototype managed to reach only {{cvt|480|km/h}}, and suffered from dangerously high engine temperatures.<ref name="Car728">Caruana 1998, p. 728.</ref><ref name="Danel 4"/>

A large portion of responsibility for the encountered temperature issue was judged to have been a product of greater than expected [[Drag (physics)|drag]] resulting from the underwing [[Radiator (engine cooling)|radiators]], which exhausted across the upper wing surface and were relatively inefficient; this arrangement was replaced with a single radiator unit housed under the [[fuselage]] in a streamlined fairing.<ref>Danel and Cuny 1966, pp. 34, 296.</ref> Other aerodynamic improvements were made around this time, such as the minor enlargement of the [[fin]] and [[rudder]], for greater lateral stability.<ref name="Danel 4"/>

After sustaining minor damage in a landing accident on 27 November 1938, caused by the failure to deploy the [[landing gear|undercarriage]], further modifications were made to the prototype.<ref name="Danel 4"/> These included changing the engine to a newer -29 model and incorporating exhaust ejectors which provided added thrust, along with a three-blade [[Variable-pitch propeller (aeronautics)|variable-pitch propeller]]. These changes were enough to allow the aircraft to reach its design speed, achieving {{cvt|530|km/h}}. The maximum diving speed was {{cvt|825|km/h}}, as reached on 6 February 1939.<ref>Danel and Cuny 1966, pp. 34–36, 297.</ref><ref name="Danel 4"/>