Why airborne early-warning radar was not ready in 1939

Why did airborne early-warning radar not mature before the Second World War?

Contemporary valves could not yet handle enough power, and war arrived before promising airborne work could mature into an operational system.

Valve power was the immediate technical bottleneck. Surface-search radar advanced first, and airborne early-warning work had begun but was not mature when Britain entered the war. The problem was therefore timing as much as concept: the hardware was moving in the right direction but had not reached operational readiness. A British entry into war in 1941 rather than 1939 might have allowed airborne early warning to appear from the outset.