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Throughout the prototype period the various models were given series designations (as noted above), and presented to the RLM as the basis for series production. The Luftwaffe never took them up on the offer. Heinkel had decided to build a total of 25 of the aircraft one way or the other, so with ten down, there were another 15 of the latest model to go. In keeping with general practice, any series production is started with a limited run of "zero series" machines, and this resulted in the He 100D-0. The D-0 was similar to the earlier C models, with a few notable changes. Primary among these was a larger vertical tail in order to finally solve the stability issues. In addition the cockpit and canopy were slightly redesigned, with the pilot sitting high in a large canopy with excellent vision in all directions. The armament was reduced from the C model to one 20 mm MG/FF-M in the engine V firing through the propeller spinner, and two 7.92 mm MG17s in the wings close to the fuselage. The three D-0 aircraft were completed by the summer of 1939 and stayed at the Heinkel Marienehe plant for testing. They were later sold to the Japanese Imperial Navy to serve as pattern aircraft for a production line, and were shipped there in 1940. They received the designation AXHe. He 100D-1
The final evolution of the short He 100 history is the D-1 model. As the name suggests the design was supposed to be very similar to the pre-production D-0s, the main planned change was to enlarge the horizontal stabilizer. But the big change was the eventual abandonment of the surface cooling system, which proved to be too complex and failure prone. Instead an even larger version of the retractable radiator was installed, and this appeared to completely cure the problems. The radiator was inserted in a "plug" below the cockpit, and as a result the wings were widened slightly. While the aircraft didn't match its design goal of 700 kph once it was loaded down with weapons, the larger canopy and the radiator, it was still capable of speeds in the 400 mph (644 kph) range. A low drag airframe is good for both speed and range, and as a result the He 100 had a combat radius between 900 and 1000 km compared to the Bf 109's 600 km. While not in the same league as the later escort fighters, this was at the time a superb range and may have offset the need for the Bf 110 to some degree. Finally, there were allegations that politics played a role in killing the He 100. By this point the war was underway, and as the Luftwaffe would not purchase the aircraft in its current form, the production line was shut down. The remaining 12 He 100D-1c fighters were used to form Heinkel's Marienehe factory defense unit, flown by factory test pilots. They replaced the earlier He 112s that were used for the same purpose, and the 112s were later sold off. At this early stage in the war there were no bombers venturing that far into Germany, and it appears that the unit never saw action. The eventual fate of the D-1s remains unknown. The aircraft were also put to an interesting propaganda/disinformation role, as the supposed Heinkel He 113. Foreign use: When the war opened in 1939 Heinkel was allowed to look for foreign licensees for the design. Japanese and Soviet delegations visited the Marienehe factory in late October, and were both impressed with what they saw. Thus it was in foreign hands that the 100 finally saw use, although only in terms of adopted design features. Six He 100s were exported to the Soviet Union and three were exported to Japan. Although any Japanese aircraft that survived the war would have been destroyed by the allies, there is a possibility that parts of or even a complete He 100 may exist somewhere in storage in Russia. It is also possible the Russians made plans or blueprints of their He 100s while the design was being studied. The Soviets were particularly interested in the surface cooling system, and in order to gain experience with it they purchased the six surviving prototypes (V1, V2, V4, V5, V6 and V7). After arriving in the USSR they were passed onto the TsAGI institute for study; there they were analyzed with He 100 features influencing a number of Soviet designs, notably the LaGG-3 and MiG-1. Although the surface cooling system wasn't copied, the addition of larger Soviet engines made up for the difference and the LaGG-3 was a reasonably good performer. It's perhaps ironic that German aircraft would later be shot down by German inspired aircraft. The Japanese were also looking for new designs, notably those using inline engines where they had little experience. They purchased the three D-0s for 1.2 million RM, as well as a license for production and a set of jigs for another 1.8 million RM. The three D-0s arrived in Japan in May 1940 and were re-assembled at Kasumigaura. They were then delivered to the Japanese Naval Air Force where they were re-named AXHei, for "Experimental Heinkel Fighter". When referring to the German design the aircraft is called both the He 100 and He 113, with at least one set of plans bearing the later name. In tests the Navy was so impressed that they planned to put the aircraft into production as soon as possible as their land based interceptor; unlike every other armed forces organization in the world, the Army and Navy both fielded complete land based air forces. Hitachi won the contract for the aircraft and started construction of a factory in Chiba for its production. With the war in full swing in Europe however, the jigs and plans never arrived. Why this wasn't sorted out is something of a mystery, and it appears there isn't enough information in the common sources to say for sure what happened. The DB 601 engine design was far more advanced than any indigenous Japanese design, which tended to concentrate on air cooled radials. To get a jump into the inline field, Kawasaki had already purchased the license for the 601A from Daimler Benz in 1938. The adoption process went smoothly, they adapted it to Japanese tooling and had it in production by late 1940 as the Ha-40. At the same time Kawasaki was working on two parallel fighter efforts, the Ki-60 heavy fighter and the Ki-61. The former was abandoned after poor test results (the test pilots disliked the high wing loading) but work continued on the lightened Ki-61 with the Ha-40 engine. The Ki-61 was clearly influenced by the He 100. Like the Ds, the Ki-61 lost the surface cooling system (although an early prototype may have included it), but is otherwise largely similar in design except for changes to the wing and vertical stabilizer. Since the Ki-61 was supposed to be lighter and offer better range than the Ki-60, the design had a longer and more tapered wing for better altitude performance. This also improved the handling and the aircraft was put into production. The Hien would prove to be the first of the Japanese aircraft that was truly equal to the contemporary US fighters.
Sources:
Gunston, Bill & Wood, Tony - Hitler's Luftwaffe
, 1977, Salamander
Books Ltd., LondonWikipedia - He 100
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