Very first country to develop the technology of mobile radar systems was the Great Britain. It was established on plane 'Avro Anson' in 1937 with coverage of approximately 1 mile for airborne targets and 3 miles to ships. On the other hand, serial production of these systems later called 'Al Mk. IV' began in 1940. They were mounted on 'Bristol Blenheim' bombers

If you are interested, here the information about other major countries of that period: •The USA received its first mobile radar in 1941 specially for 'Douglas P-70' fighters, but the actual serial production started in 1942 for 'Northrop P-61 Black Widow' night fighters. •The USSR developed its first prototype in 1941 which was then fully developed only in 1942, where was mounted on 'Pe-2' fighters in Stalingrad battle •Germany started testing mobile radar systems in 1941. But the fully operational model was deployed only in 1942 for Ju-88 night fighters •Japan's first mobile radars were introduced in 1942, but their mass production began in 1944. They were first mounted on H8K 'Emily' flying boats.