What aircraft had the first pilot-assisted controls?
It was the Lockheed Constellation and the P-80, both introduced in 1943. The DC-3 had hydraulic flaps, but the Connie was the first to have hydraulic flight controls.


  The DC‐3 had wings flaps powered by hydraulics (1934). But it wasn’t until the early 1940s that hydraulics were used to power the primary flight control surfaces on the Lockheed Constellation and P‐80 (first US Jet Aircraft).


Source: A History of Aviation Actuation, Control and Fluid Power (PDF)

Also in 1943, the Lockheed P-38J-25-LO version of the WWII fighter aircraft was fitted with hydraulic boosted ailerons.

Source: Lockheed P-38J Lightning, Joe Baugher