The formation of the Heston Flight was on 23 September 1939 it was decided to form a unit to investigate and develop techniques of photographic reconnaissance over enemy territory. The unit was to be a part of 11 Group, Fighter Command. Based at the civilian airfield, Heston, the hope being that the work undertook there would be kept a secret.  Under the leadership of the newly commissioned Squadron Leader (acting Wing Commander) Sidney Cotton, the flight was equipped at the very start with Cotton's Lockheed and Beechcraft aircraft, they later received two Blenheim IVs, these were sent away for modification to undertake reconnaissance missions. Still Cotton felt they need newer aircraft, so after a meeting with Air Chief Marshal Dowding, the then Commanding-in-Chief of Fighter Command, the Unit received two Spitfire Mk1s, N3069 and N3071. These were converted to Spitfire PR Mk 1As.
To conceal its true identity the Heston Flight was renamed No.2 Camouflage Unit on the 1st November of that year. Due to the lack of the range of the Spitfires, which was still the same as the fighter version of only 650 miles, so on the 5 November, the Special Survey Flight was formed from the main unit and detached from Heston to Seclin airfield near Lille in France. Equipped with just one Spitfire PR 1A, two pilots and eight groundcrew, the aircraft now had the range to over fly German territory. The flight was again moved on the 20 November, this time they transferred to Coulommiers airfield 30 miles east of Paris, again to keep the unit away from the attention of the enemy.

Due the amount of work and the high success rate of the Special Survey Flight, by January of 1940 the flight had flown 15 missions and covered 5000 square miles, both No.2 Camouflage Unit and the Special Survey Fight were renamed to The Photographic Development Unit. This changed the units role from one of experimental to that of an operational unit. To that end, the PDU was given two tasks, to supply reconnaissance in support of Allied operations within France and maintain a watch on the German fleet and its bases. Equipped with now Spitfires PR Mk 1Bs they could undertake longer range sorties.
Since the beginning Wing Commander Cotton had been in overall command of the development of the RAF photographic reconnaissance unit, however, the Air Ministry decided that the PDU would come under the operational control of RAF Coastal Command as of the 18th July 1940 and have a new commanding officer, Cotton was handed a letter saying that he had been dismissed and was to be replaced by Wing Commander Geoffrey Tuttle.  Now the unit was fully under RAF command and with a regular RAF Officer in charge.