The answer would be that Germany's economy was not devastated physically during WW1, it was merely drained of finance and materials. Indeed the lack of devastation to Germany itself contributed to the stabbed in the back theory.

Now while Germany's economy did suffer during the 1920's and early 1930's the heavy industry needed for war wasn't destroyed or dismantled and was supported by foreign loans. The Ruhr was still the coal and steel production capital of Europe.

Now after 1933 the German economy was given a massive cash injection with rearmament which allowed it to expand and rebuild the Germany military. Combine this with expert designers, cutting edge technology and a delay in their opponents response by 1939 German held a short term technological advantage.

Hope that gives you some ideas