The P-51 was regarded as the best overall fighter of the World War II warplanes in terms of performance. Unlike the U.S. Army's policy of using turbochargers for both liquid- and air-cooled engines to enhance high-altitude performance, the P-51 was not a U.S. Army aircraft and could not obtain a turbocharger quota at the beginning of the design period; therefore, North American used the same domestic Allison liquid-cooled engine as that of the P-40, which was equipped with a one-stage supercharger, as the source of power. However, compared with the P-40, the aircraft had a more advanced intake configuration and overall layout to greatly improve cooling and aerodynamic efficiency, and incorporated a laminar flow wing design to further enhance high-speed performance. The conversion to the British Merlin engine with a two-stage, two-speed mechanical supercharger improved the aircraft's high-altitude performance, and the North American company modified the new engine to address the problems it caused, thus achieving its rare historical rating.Before the P-51D appeared, the range of fighters was relatively short, and the heavyweight P-47s and P-38s were not ideal for escort duties due to range limitations and engine technology problems. Bombers had to maintain their bombing formations in the 1/3 round trip approaching the target area without any escorts, against a rain of German anti-aircraft gunfire and fighters chasing them. In the so-called "Black Thursday", up to 100 of the 330 bombers were shot down, and many others were damaged and scrapped on their return to England. With the P-51's escorting the Luftwaffe fighters, the threat from the Luftwaffe's fighters was greatly reduced in favour of the bomber units carrying out their bombing missions against targets that would devastate Germany. Hermann Goering said in a post-war interview, "When I saw the P-51s over Berlin, I knew it was over!"