:Please check the article on Ferdinand Porsche, where this is briefly covered. Porsche was mainly an engineering design firm at the time, they did not have the manufacturing capacity to mass produce cars, much less tanks. The truth is however, that Hitler held Porsche's engineering talent in very high esteem, which led to a number of defense contracts. Actual manufacturing might have occurred at a number of large industrial contractors such as MAN, Daimler (where Ferdinand started his career), Krupp and others. 

 :Also, check the Tiger I, Tiger II and Elefant articles for further info. The value of the Elefant in combat is being disputed and it's quite likely that Porsche's contribution to the design of Tiger I, Tiger II and the Kuebelwagen (Nazi equivalent of the Jeep) might had more impact on the battlefield.