The first lessons learned by Stuka outfits during the Polish campaign clearly demonstrated to the Luftwaffe command that Junkers Ju 87 was in need of a major upgrade. Junkers engineers were directed by Technical Department officials to find ways to improve Ju 87’s fire power and to increase its bomb load, operational range and speed performance in level flight.
The key elements in achieving those goals appeared to be the use of a more powerful engine and aerodynamic modifications of the airframe.
Junkers Ju 87 D (Dora)
The initial work on the new Stuka model began in the spring of 1940. In May 1940 the new variant received its official designation: Junkers Ju 87 D. Originally the aircraft was to be equipped with a Jumo 211 F engine, but delays in its development sent Junkers engineers searching for an available replacement. They chose Jumo 211 J-1 powerplant, which was also being developed at that time. It was a 1,420 Hp, twelve cylinder inline unit with liquid cooling. Because the new engine was as much as 40 cm longer from its predecessors, the cowling had to be lengthened and redesigned. An improved engine cooling system was also introduced at the same time. Oil radiator was placed under the lower part of the engine fairing and two engine coolant radiators were added under the wing’s center section, near the trailing edge. The aircraft was also fitted with a new cockpit canopy, which was tested on a Ju 87 B (W.Nr. 2291).
Jumo 211 J-1 engine was mated to a Ju 87 B-1 airframe (W.Nr. 0321, D-IGDK) in October 1940. Flight testing of that configuration lasted for several weeks and was marred by frequent malfunctions of the powerplant. Ju 87 V21 (W.Nr. 0536, D-INRF), the first prototype of Ju 87 D model, was rolled out in March 1941. Flight test program of the Jumo 211 J-1 powered aircraft was carried out at Dessau between March and August 1941. In August 1941 Jumo 211 J-1 engine was replaced by a Jumo 211 F powerplant. During one of the early flight test sorties of the new powerplant, an in-flight separation of the propeller occurred at 1,420 rpm setting. After repairs, the aircraft was transferred to Rechlin test and evaluation facility on September 30, 1939. After another series of tests the machine was officially accepted by the Luftwaffe on October 16, 1941. The aircraft was subsequently used as a test bed for powerplant and cooling system tests. In February 1942 the bomber returned to Dessau for installation of new radiator fairings. The prototype was eventually taken on strength by a frontline unit on September 14, 1943.
The second prototype, Ju 87 V22 (W.Nr. 0540, SF+TY) was originally scheduled to be completed by the end of 1940. However, due to problems with the powerplant, the aircraft was not ready to begin its flight test program until May 1941. It was handed over to the Luftwaffe on November 10, 1941. Flight test program results were so far satisfactory both to Junkers staff and to personnel at Rechlin. Thanks to unusually cold November of 1941, some cold-start engine trials could also be run, which demonstrated that engine start procedures, even in extremely low temperatures, were straightforward and did not cause any malfunctions of the powerplant.
The prototype returned to Dessau at the beginning of 1942 for additional stability testing and some minor modifications of its Jumo 211 J-1 engine, following which it was sent back to Rechlin. On August 20, 1942 the aircraft was lost during one of the test sorties when it crashed into lake Müritzsee. The crew, Fw. Hermann Ruthard and a civilian employee of Rechlin test center, were killed in the crash. Probable cause of the accident was carbon monoxide poisoning of the crew and their subsequent loss of consciousness.
The third prototype, Ju 87 V23 (W.Nr. 0542, PB+UB) was completed in April 1941 and sent to Rechlin a month later. That particular example was a benchmark for Ju 87 D-1 production model. Continuing delays in Jumo 211 J-1 deliveries grounded the next prototype: Ju 87 V24 (W.Nr. 0544, BK+EE) could not be finished before August 1941. Upon its completion, the aircraft was promptly sent to Rechlin. The prototype was soon returned to Dessau after an accident in which it suffered damage to the fuselage. After repairs, the machine was once again delivered to Rechlin test and evaluation center in November 1941. Having completed the flight test program, the bomber was handed over to a combat unit.