Early radar experiments versus operational airborne warning

Did airborne early-warning development begin before the Second World War?

Yes. Visual warning and radar experiments preceded the war, while radar-equipped Avengers provided an operational warning force at its end and afterwards.

Airborne warning has earlier roots, including observers using binoculars in the 1920s and radar development before the Second World War. A Swordfish anecdote describes a surface-search radar detecting a reconnaissance aircraft and passing information back to a carrier so its fighters could intercept. The location is tentatively remembered as the Indian Ocean and left for checking. That encounter is treated as a fortunate detection rather than an established airborne early-warning system. Radar-equipped Avengers entering service in numbers provide the example of a dependable force capability at the end of the war and afterwards.