Mk III converted to IV
During the summer of 1943, two prototype Stirling IVs were flown, including the glider-tug EF506 and the paratroop LJ512, which were converted from Rochester-built Stirling Ills.
The glider-tug version was essentially a Stirling III with the front and dorsal turrets removed and a standard M.L. glider-towing coupling installed on a stirrup around the tail turret, which was sometimes removed but often left in place. On the other hand, the paratroop version had a glazed cupola in place of the tail turret and no towing gear, but it had a large hatch in the floor of the rear fuselage aft of the bomb cells for the paratroopers to exit. After acceptance trials of the prototypes, most of the remaining Stirlings on the Rochester production line were completed as Mk IVs. The first 12 were delivered to the RAF during the last three months of 1943. Production of both versions of the Stirling IV increased to a steady rate of 30 to 35 per month and continued until mid-1945. During this time, 145 were built at Rochester, 236 at Belfast, and 198 at Longbridge.
As a glider-tug, the Stirling IV was approved to tow one Hamilcar or two Horsas for assault, or up to five Hotspurs for ferry or training purposes. On D-Day (June 6, 1944), Stirling IVs of No. 38 Group (Allied Expeditionary Air Force) were first in action when Nos. 196 and 299 Squadrons at Keevil and Nos. 190 and 622 Squadrons at Fairford towed Airspeed Horsa gliders to their dropping zones behind the Normandy beachheads. They also participated in the Arnhem and Nijmegen operations in September and in the final invasion assault across the Rhine in March 1945. Their duties were not limited to assault aircraft as they also supplied food and ammunition to paratroops in the front line and fuel to squadrons of the 2nd Tactical Air Force. From D-Day to VE-Day, they were continuously engaged in maintaining these supplies, carrying a regular load of 120 five-gallon drums of petrol in each aircraft.
Stirling Ills and IVs of Nos. 138 and 161 (Special Duties) Squadrons were flown under the Special Operations Executive from Tempsford (and No. 624 similarly from Blida in North Africa) to supply arms and equipment to agents and partisans in occupied areas. Later, they joined with Transport Command Squadrons Nos. 295, 570, and 620 in general supply train operations. Stirling IVs used as glider-tugs in the later actions had fan-cooled engines to reduce overheating at low cruising speeds.