Pilot Director Indicator as used with C-1 Autopilot, WWII B-29, B-24, B-17 (B)
The Pilot Director Indicator (PDI) was used during WWII in bombers of the US Army Air Force, including heavy bombers B-17 Flying Fortress, B-24 Liberator, and B-29 Superfortress and medium bombers B-25 Mitchell and B-26 Marauder, and US Navy's PBY5A Catalina and PB4Y Privateer.
The PDI was located on the pilots instrument panel, and received signals from the bombardier to indicate his desired course changes, left or right, to obtain an accurate bomb run. It was used in aircraft with the C-1 Autopilot system, or in those without the C-1, it received its signals from a Type A-1 pilot indicator director control. See the spec sheet from 1944.