The first aircraft adopted for the Schnellbomber role was the single-engined Heinkel He 70, but it soon was replaced by the twin-engined Dornier Do 17 in that role. In the 1937 air races in Switzerland the Do 17 won a number of speed records, apparently demonstrating the value of the concept. However, experience of the Spanish Civil War demonstrated that the Do 17's speed when loaded with military equipment was insufficient to escape interception, and armament had to be added to give it some defensive fighting capability.