What was the first aircraft equipped with a RAT? RAT are common in airliners today to provide  emergency electrical and hydraulic power. Yet, firsts airliners (prior to WWII) may not require the same amount of power than today's airliners (which are bigger and require electrical and hydraulic power to keep all electric-assist requirements, computers, hydraulic actuators,...).

The historic section of the wikipedia article speak about windmilling on the FW-189's engine as a precursor to RAT, but I fail to see anything related to first device whose only role is provide power.

My question is: What was the first aircraft equipped with a RAT? Was it military or civilian (it may not be a airliner)? 

EDIT: given the comment by Peter Kämpf, I edited the question to include all aircrafts. Thus having an answer mentioning both airplanes and other aircrafts.
So far, the oldest I have found is LZ-127 Graf Zeppelin, first flight in 1928. It used a propeller-driven generator to power its radios and a second one (with buffer battery) to keep the ship illuminated. In the drawing below (source) they are indicated as "Wind-Dynamo", being on both sides of the main gondola.

I am rather sure that the wartime Zeppelins used those generators as well, but have yet to find proof for it.





LZ-127 on August 26, 1929, in Los Angeles (source). The right generator is circled in red.

EDIT: Now I have found a source that attributes a RAT to the Zeppelin Staaken Riesenflugzeuge of WW I. The source is Tom Morrison's book "Quest for All-Weather Flight" and it says on page 28:


  In November 1917 the Drexler bank indicator was introduced instead [of the Anschütz gyro]. Much simpler than the Anschütz instrument, it weighed only 5 kg (11 pounds). It was powered by electricity from a generator driven by a small propeller in the slipstream and took less than a minute to run up to speed.


So this dates the first RAT back to November 1917. Sorry, no online sources.