The unit was formed at RAF Tangmere in April 1940 under the command of Squadron Leader George Philip (Peter) Chamberlain, with a strength of 5 Blenheims equipped with the latest AI Mk III radars.
Leyland reported a response on the AI at a range of 8,000 feet and presently P/O Morris made a visual sighting of a Dornier Do 17 to port and below the Blenheim.
A two-seat night fighter version – the Welkin NF Mk II – was also evaluated but only two were produced was not ordered into production.
A radar-equipped Vickers Wellington was modified for use by the Fighter Interception Unit as one of the first Airborne Early Warning and Control (AEW&C) aircraft.
The modus operandi typically involved the Heinkels leaving bases in the Netherlands and flying out over the North Sea at a height of less than 300 feet.
Once the Heinkels neared the East Anglian coast they would increase speed and release their flying bombs before turning for home at low level.
To assist in detecting the Heinkels the Fighter Interception Development Squadron borrowed a Coastal Command Wellington equipped with a modified ASV Mk VI radar set and PPI to act as Airborne Early Warning and Control.