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 Müller served in the No. 331 Squadron RAF, when he was shot down over the sea during a flight mission on 19 June 1942. He was captureed and brought a prisoner-of-war camp, Durchgangslager Frankfurt, and after interrogations eventually transferred to the camp Stammlager Luft III. As a prisoner in the camp, Müller participated in the escape plans, by helping with the ventilation of the tunnels dug by the prisoners. In 1943 the prisoners were moved to a new camp site, and shortly thereafter they secretly organized the digging of three new tunnels, called Tom, Dick and Harry. On 24 March 1944 a number of prisoners made their way out of the camp through the tunnel Harry. Along with fellow prisoner Per Bergsland from RAF 332 Squadron, Müller eventually managed to escape to Sweden, after travelling with train to Frankfurt, taking a connecting train to Küstrin, then another train to Stettin. During the travel they were camouflaged as Norwegian electricians working in Germany, equipped with false documents made by other prisoners, including travel orders to change their workplace from Frankfurt to Stettin. They finally reached Sweden by hiding on a ship. After about ten days in Stockholm they were transported to Scotland in two Mosquito aeroplanes (each Mosquito could only carry one passenger), and arrived in London on 8 April 1944. Only three of the seventy-six runaways from Stalag Luft III managed to escape to freedom, while the other 73 were caught after a giant man-hunt, and fifty men where executed by shooting as reprisal. numbers that have been corrected after later investigations.