y ' ▁Bow en ▁in ▁the ▁mid - 1 9 3 0 s . ▁In ▁September ▁ 1 9 3 7 , ▁he ▁gave ▁a ▁working ▁demonstr ation ▁of ▁the ▁concept ▁when ▁a ▁test ▁aircraft ▁was ▁able ▁to ▁detect ▁three ▁Home ▁Fle et ▁capital ▁ships ▁in ▁the ▁North ▁Sea ▁in ▁bad ▁weather . ▁ ▁The ▁prom ising ▁imp lications ▁of ▁the ▁test ▁were ▁not ▁lost ▁on ▁pl ann ers , ▁who ▁re organ ized ▁rad ar ▁efforts ▁and ▁gave ▁them ▁increased ▁priority . ▁This ▁led ▁to ▁efforts ▁to ▁develop ▁an ▁oper ational ▁unit ▁for ▁air bor ne ▁inter ception ▁( AI ). ▁The ▁size ▁of ▁these ▁early ▁A I ▁rad ars ▁required ▁a ▁large ▁aircraft ▁to ▁lift ▁them , ▁and ▁their ▁complex ▁controls ▁required ▁a ▁multip erson ▁crew ▁to ▁operate ▁them . ▁This ▁naturally ▁led ▁to ▁the ▁use ▁of ▁light ▁bom bers ▁as ▁the ▁preferred ▁platform ▁for ▁air bor ne ▁rad ars , ▁and ▁in ▁May ▁ 1 9 3 9 , ▁the ▁first ▁experimental ▁flight ▁took ▁place , ▁on ▁a ▁F aire y ▁Battle . ▁ ▁World ▁War ▁II ▁ ▁The ▁war ▁opened ▁on ▁ 1 ▁September ▁ 1 9 3 9 , ▁and ▁by ▁this ▁time , ▁the ▁R AF ▁were ▁well ▁advanced ▁with ▁plans ▁to ▁build ▁a ▁rad ar ▁– ▁then ▁called ▁' R DF ' ▁in ▁Britain ▁– ▁equ ipped ▁night - fig h ter ▁fleet . ▁The ▁Air bor ne ▁Inter ception ▁M k . ▁II ▁rad ar ▁( AI ▁M k . ▁II ) ▁was ▁well ▁on