This a complete myth. The German Economy was never a strong economy, and was always on the verge of one crisis after another. For the definitive and best book out there on the Nazi economy, you must read Adam Toozes’s “The Wages of Destruction.” No book even comes close to grappling with the Nazi Economy that this book does - http://www.amazon.com/Wages-Destruction-Making-Breaking-Economy/dp/0713995661.
The Nazi economy was a centrally planned economy, and thus a house of cards.