The concept underlying the X-20 was developed in Germany during World War II by Eugen Sänger and Irene Bredt as part of the 1941 "Silbervogel" proposal. This was a design for a rocket-powered bomber able to attack New York City from bases in Germany and then fly on for landing somewhere in the Pacific Ocean held by the Empire of Japan. The idea would be to use the vehicle's wings to generate lift and pull up into a new ballistic trajectory, exiting the atmosphere again and giving the vehicle time to cool off between the skips. After the war, it was demonstrated that the heating load during the skips was much higher than initially calculated, and would have melted the spacecraft.