Even before the 109 had entered squadron service, in autumn 1937 the RLM sent out a new tender asking various designers for a new fighter to fight alongside the Bf 109, as Walter Günther had done with his firm's follow-on to the unsuccessful He 100 and He 112. Although the Bf 109 was an extremely competitive fighter, the Ministry was worried that future foreign designs might outclass it, and wanted to have new aircraft under development to meet these possible challenges. Kurt Tank responded with a number of designs, most based around a liquid-cooled inline engine.