Aviation of World War II
High Altitude Bomber
In 1934, the Junkers Flugzeugwerke sent a proposal to the Technical Department of the Air Commissariat for a pressurized bomber capable of operating at heights inaccessible to fighters. This radical proposal aroused keen interest, and after Junkers proposed the EF-61 project in early 1935, an order was issued for two prototype aircraft. Meanwhile, flight tests of the Ju.49 continued, but after reaching an altitude of 12,500 m in September 1935, the engine stalled and the aircraft was wrecked.
The first of two experienced high-altitude bombers EF-61-V1 was ready in the fall of 1936. It was equipped with two Daimler-Benz DB-600A engines with a take-off power of 950 hp with frontal radiators. The first flight took place at the end of the year. The aircraft had a two-spar wing of high elongation, with a span of 27 m. The fuselage had an oval section with a corrugated working skin covered with fabric. The nose was formed by a round section of a pressurized cabin for two crew members. The view of the crew was provided by a large round panel in the bow of the cabin. Glazing was double with a layer of dried air. The glazing material was the so-called "reilite". A small round panel on the lower, right side of the cockpit provided a downward view of the pilot during landing. A transparent panel was also in the floor on the left for aiming the scorer, who acted as a navigator and shooter. In the latter case, he could use the MG-15 machine gun in the rear of the cockpit. The cabin was accessed through a round hatch on its right side.
Delays in the readiness of pressurized cabin forced to start flying without it. However, the plane never received it. On September 19, 1937, it hit a flutter and crashed, killing both pilots. While the plane was flying around, the pressurized cabin was also tested for pressure. As a result, it turned out that the plastic was too fragile for glazing a large round panel and could not withstand pressure at an altitude above 12,000 m. Thus, work on the second experimental EF-61-V2 was also delayed: its first flight took place only in October 1937.
The new pressurized cabin without large area glazing and portholes received only one transparent blister, shifted to the starboard side. The pilot's seat was raised and moved to the very end of the cockpit. Above the pilot's seat was a metal dome with two plexiglass portholes, providing a view forward and to the left. At the end of the blister, a MG-15 machine gun was installed. Behind the cockpit was equipped with a small bomb bay for four 250 kg bombs.
In addition to the pressurized cabin and some changes to the engine nacelles, the V2 did not differ much from its predecessor. Unfortunately, even before the start of high-altitude flights, the second plane crashed in December 1937. Since the first tests were disappointing, the Technical Department decided that serious fine-tuning of the pressurized cabins would be required. There were no plans to build a new aircraft. Thus, work on the first high-altitude bomber with a pressurized cabin was discontinued.
Although the EF-61 prevented the Luftwaffe from obtaining a high-altitude bomber that was inaccessible to interceptors, it gave the German aircraft industry experience in working with pressurized cabins when they had not yet reached practical implementation in other countries. One can only surmise what success similar work would have achieved in England, France and other countries, had they had such experience.
16 months before the termination of work on the EF-61 pressurized cabin developed by the Bureau of Special Designs under the leadership of V.A. Chizhevsky in the USSR was installed on a modified ANT-25, which already flew under the designation BOK-1. Only 18 months after the accident of the second EF-61, work began on similar machines in England and the USA. In the UK, requirements B.23 / 39 were prepared for the Wellington high-altitude bomber with a pressurized cabin. At the same time, the US Army Air Corps began preparing requirements for a pressurized bomber. As a result of the latter, the North American XB-28 appeared. None of these aircraft went into battle.