**No, he did not design it**. It was designed by Ferdinand Porsche.
It was however commissioned by Hitler. Hitler met with Porsche in 1933 and asked him to develop a cheap good car affordable for everybody, a "Peoples Car". Porsche was a natural choice for this, because he in fact already had developed such a car, the Porsche Type 12, produced by Zündapp from 1931.
!Porsche Type 12
In 1932 he produced the Porsche Type 32, a car quite strikingly similar to the Volkswagen, including having a 4-cylinder air cooled engine.
!Porsche Type 32
Claiming that Hitler designed the Beetle is hence nonsense, as Porsche's Type 32 has most of the striking design and many of the Beetle's technical elements in place, well before Hitler and Porsche ever met, and **looks more like a Beetle than the alleged sketches by Hitler**.
Further reading, and sources:
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