Bomber B
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Bomber B was a German project dating to just before the start of World War II to develop a second-generation high-speed bomber that would replace all medium and heavy bombers then in service with the Luftwaffe. The RLM was so hopeful about the outcome that more modest projects were generally cancelled outright, so when the project eventually failed to deliver a working design the Luftwaffe was left with hopelessly outdated aircraft.
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Background
The main problem for aircraft designers in the 1930s was a lack of engine power. Construction methods had progressed to the point where airframes could be built at any required size, but the engines needed to lift them were not available. The US, confident in its ability to produce any required quantity of engines, opted for four-engine designs with heavy defensive firepower, as seen in the B-17. England and Germany did not have this luxury, both were severely constrained in engine production and tried to make do with two-engine designs. Both also invested heavily in a new generation of much more powerful engines, which would provide the needed power for B-17-sized aircraft powered by only two engines. By the late 1930s the new high-power engines started to run, and both England and Germany drew up designs based on them. In England the Avro Manchester proved to be a failure, but when the Royal Air Force finally acquiesced and the design was re-built with four Rolls-Royce Merlin engines, the resulting Avro Lancaster emerged as possibly the first truly useful heavy bomber.
Schnellbomber
In 1936 the RLM had started the competition for the first purpose-designed schnellbomber, a bomber fast enough to simply outrun the defending fighters. It was thought that, with the limited power available, the bomber's two engines would mean it would always be faster than a single-engined fighter. This point was "proven" in the 1936 Air Races in Zürich, where the Dornier Do 17's outran fighter designs from across Europe.
Later that year they selected the Junkers Ju 88 over a number of competitors to become their first purpose designed schnellbomber. However the tradeoffs made for speed were serious, the Ju 88 was a small aircraft, carrying a fairly small load in its tiny bomb bay. This could be addressed by adding additional load on external racks, but doing so dramatically reduced performance due to increased drag. In addition the limited engine power meant that the plane could not lift a reasonable warload and the fuel needed to carry it any distance, dramatically limiting its combat effectiveness. In order to make up for its shortcomings, a number of other aircraft such as the Heinkel He 111 and Dornier Do 17 were also pressed into service to fill particular performance niches, a problem no one in the Luftwaffe was at all happy with.
The Ju 88 was just entering service when Germany's own high-power engines started bench testing. Daimler-Benz was offering the DB 604 and Junkers their Jumo 222, both of which planned on delivering 2,500 hp (1840 kW) to start with. Compared to the Jumo 211's in the Ju 88, these new engines would over double the power available to 5,000 hp (3680 kW). With this sort of power a significantly more capable design could be built, one with considerably larger internal space for a much large bombload, more fuel for longer range, and even better speed.
The Bomber B project
Junkers had been studying dramatically more capable versions of the Ju 88 powered by their Jumo 222 or Jumo 223 from late 1937. No serious work was undertaken, but after Heinrich Hertel left Heinkel and joined Junkers in 1939, the EF 74 design was submitted to the RLM in May 1939. Accordingly the RLM sent out the specifications for Bomber B in July 1939, the Ju 88 retroactively becoming Bomber A. Bomber B called for a new medium with a maximum speed of 600 km/h (375 mph), able to carry a bomb load of 4000 kg (8,820 lb) to any part of Britain from bases in France or Norway. To improve crew performance and defensive firepower, the designs were to have a pressurized cabin with remote control armament. With the extended range, larger payload and better performance, the Bomber B design would replace all existing bombers in service.
Arado, Dornier, Focke-Wulf and Junkers all responded with designs, and Henschel later added an entry as well. However it was clear even at this point that the call for designs was to some extent a formality, the Junkers design had already been selected for production. The Ar 340 was dropped in the design stage and Do 317 was put on low-priority development, while prototype orders were placed for the Fw 191 and newly-named Ju 288. With the Focke-Wulf and Dornier projects as first and second backups, the T-Amt started using these other designs as experimental testbeds. For instance, as the aircraft would be operating at high altitudes, they suggested that all hydraulic systems on the Fw 191 be replaced with electrical ones instead, to avoid the possibility of freezing up. However this dramatically increased the complexity of wiring the planes, and the chance that one of the many motors would fail was considerable. But that was not terribly important -- the Junkers design would work anyway.
The end of the project
Prototype airframes of both designs were ready mid-1940, but in a taste of things to come, neither the Jumo or DB engines were ready to be installed. Instead of waiting, both teams decided to power their prototypes with the BMW 801 radial engine, although with 900hp less per engine, the planes would be seriously underpowered. The first 222's did not arrive until October 1941, and by this point the DB project had already been cancelled. By May 1942 things were getting desperate, and it was suggested that the Daimler-Benz DB 606 be used instead, even though it was considerably larger and heavier. Prototypes of both designs with these engines were ordered, although the Fw was just getting into the air with the 801s at this point and the 288 was showing a continual tendency to break its landing gear on touchdown. Desperation set in at the RLM, who had no other designs in the pipe to fill the gap left if Bomber B didn't work. A slightly improved Ju 88 was ordered as the Ju 188, and several prototypes of "stretched" versions of existing bomber designs with four engines were also ordered.
In June 1943 the T-Amt finally gave up; by this point even if the Jumo 222 started working reliably, as it had been recently, a shortage of the metals needed for the high-temperature alloys it used meant it wouldn't be able to enter production anyway. The Bomber B project was a massive and amazingly costly venture that delivered nothing, while also serving to ensure that no other designs were available in the late-1943 time-frame when their existing planes started to become hopelessly outdated.