F4U-1D (Corsair Mk IV): Built in parallel with the F4U-1C, but was introduced in April 1944. It had the new -8W water-injection engine. This change gave the aircraft up to 250 hp (190 kW) more power, which, in turn, increased performance. Speed, for example, was boosted from 417 miles per hour (671 km/h) to 425 miles per hour (684 km/h). Because of the U.S. Navy's need for fighter-bombers, it had a payload of rockets double the -1A's, as well as twin-rack plumbing for an additional belly drop tank. Such modifications necessitated the need for rocket tabs (attached to fully metal-plated underwing surfaces) and bomb pylons to be bolted on the fighter, however, causing extra drag. Additionally, the role of fighter-bombing was a new task for the Corsair and the wing fuel cells proved too vulnerable and were removed.
The extra fuel carried by the two drop tanks would still allow the aircraft to fly relatively long missions despite the heavy, un-aerodynamic load. The regular armament of six machine guns were implemented as well. The canopies of most -1Ds had their struts removed along with their metal caps, which were used — at one point — as a measure to prevent the canopies' glass from cracking as they moved along the fuselage spines of the fighters. Also, the clear-view style "Malcolm Hood" canopy used initially on Supermarine Spitfire and P-51C Mustang aircraft was adopted as standard equipment for the -1D model, and all later F4U production aircraft. Additional production was carried out by Goodyear (FG-1D) and Brewster (F3A-1D). In Fleet Air Arm service, the latter was known as the Corsair III, and both had their wingtips clipped by 8" per wing to allow storage in the lower hangars of British carriers.