Operations over Dunkirk in May/June 1940 had revealed the Spitfire's limited radius of action, so Supermarine looked to extend it's fighter's range through the provision of an external tank.
Spitfire I P9565 (below) flew with a 30 gallon tank fitted below its port wing in the summer of 1940.
But the advent of the Battle of Britain stymied development until fighter command commenced offensive operations into Europe in early 41.


A small number( approx 60) of Mk IIs were converted to "Long Range" Spitfires in early 1941. These could be recognised by the fixed 40 gal (182 l) fuel tank which was fitted under the port wing. With a full tank manoeuvrability was reduced, maximum speed was 26 mph (42 km/h) lower and the climb rate and service ceiling were also reduced. Several squadrons used this version to provide long-range bomber escort.
Regards Duggy