This question about regulations regarding hydraulic flight controls got me thinking about the history of hydraulic or pilot-assisted controls. As aircraft get larger the control surfaces must grow proportionally but it seems that the forces required to move those surfaces may grow at a non-proportional rate. That means at some point they had to switch from a pure cable/pulley system to an assisted system.
What is the first aircraft to use an electrical/mechanical/hydraulic system to assist the pilot with the control surfaces? I'm mostly interested in an answer from a commercial aircraft standpoint.