The German involvement in the development of a workable design is actually quite late, as the following wiki extract shows:
The ancestor to the modern minigun was made in the 1860s. Richard Jordan Gatling replaced the hand cranked mechanism of a rifle-caliber Gatling gun with an electric motor, a relatively new invention at the time. Even after Gatling slowed down the mechanism, the new electric-powered Gatling gun had a theoretical rate of fire of 3,000 rounds per minute, roughly three times the rate of a typical modern, single-barreled machine gun. Gatling's electric-powered design received US Patent #502,185 on July 25, 1893.[1] Despite Gatling's improvements, the Gatling gun fell into disuse after cheaper, lighter-weight, recoil and gas operated machine guns were invented.
During World War I, Germany was working on the Fokker-Leimberger, an externally-powered 12 barrel Gatling gun in the 7.92x57mm Mauser round capable of firing over 7,000 rpm, but its spent brass ruptured.[2] None of the guns became operational during the war except the Siemens example which was tried on the Western Front with a victory using it during air combat. However, the Fokker-Leimberger was used in development of what eventually became the Minigun
I am not sure why yet, but i smell significant problems with this basic concept. There appear to be problems with the German development, evidenced in no small way by its virtual abandonment after the war. Weapons manufactureres do that for a reason.....usually ther are insurmountable problems that cause a project to be abandoned.
In any event the first successful minigun did not appear until 1962.....