A prototype Wellington bomber with up-to-date Bristol Hercules engines which made its first flight in May 1939. This model was launched for full-scale production at the beginning of 1941 under the designation of Wellington Mk.III (Type 417).
The Mk.III aircraft were equipped with a 1,375 hp Bristol Hercules Mk.III (though some places were equipped with the Hercules Mk.XI) fourteen-cylinder double-row radial air-cooled engine.
When working on the Wellington Mk.III bomber, the designers relied heavily on the combat experience pilots had amassed up to that time and aimed to remove the shortcomings previous models had demonstrated in combat. The defensive armament was upgraded yet again. The Frazer-Nash FN-5A rear turret with two 7.7mm machine guns was replaced with a Frazer-Nash FN-20 turret holding four 7.7mm machine guns with 2,000 rounds each.
The take-off weight of this model was increased to 15,650 kg while bomb capacity was reduced to 4,000 lbs (1,814 kg). Originally, the bomber was to be equipped with a ventral turret, the low-drag Frazer-Nash FN-21A, but the further decrease in bomb capacity was unacceptable for the RAF.
The Mk.III had its armor reinforced, and all fuel tanks were self-sealing. Barrage balloon cable cutters were positioned in the leading edge of the wings.
By 1941, the Wellington was used exclusively at night and, therefore, attention was given to decreasing the area of the glazing that spoiled the aircraft's camouflage during night flights. Moreover, night bombers, whenever possible, needed to avoid using parts that could reflect searchlight beams. Therefore, the Mk.III differed from the later Mk.IC aircraft in that it did not have any side glazing and the side blisters of the bombardier/navigator's cockpit were completely removed. The only side glazing that remained was from the trapezoidal gun blisters.
The Wellington Mk.III was manufactured until the end of 1943, and a combined total of 1,519 planes were produced.