Why These 2 Planes Were Stuck Together

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In early 1944, a British reconnaissance Spitfire pilot on patrol over Northern France reported on seeing what appeared to be two German Heinkel 111 bombers joined together as one aircraft. At first, British intelligence dismissed this as an optical illusion caused by a pair of enemy bombers flying together in close formation or the pilot just having an overactive imagination.
However, what the pilot had seen was a Heinkel 111z on a training flight. A few weeks later, an RAF Mosquito shot one down finally confirming that such a bizarre plane did actually exist.
The Giant
The Heinkel 111z had been developed back in 1941 as an aircraft that would have enough power to tow the newly introduced Me-321 heavy cargo glider off the ground and into the air. This new glider had been named โThe Giant.โ
The intention was that this extra-large glider would be utilized in Operation Sea Lion, the invasion of Britain in September 1940. However, after the Battle of Britain, when the RAF managed to retain dominance of the skies over the German Luftwaffe, this idea was abandoned. Nevertheless, it was decided by the German high command to continue the project as it could be highly useful in supplying and transporting their troops on the Russian Front.
Heinkel came up with a practical solution by joining two of their He-111 twin-engine medium bombers together. Where they were connected was an added extra engine making five in total, and boosting its power.
New, Hybrid Aircraft
The development of this hybrid aircraft was quick and relatively hassle-free. It was ready to enter service in 1942. However, it soon became clear that despite its engines having a combined power output of over 6,500 hp in total, it proved to be badly underpowered when trying to take off towing a fully laden Me-321, thus requiring modifications.
Surprisingly, in service, it was well-liked by its crews and it had an incredibly long range of over 1200 miles. This could be further extended by using fuel drop tanks allowing it to stay in the air for up to 10 hours. However, one major drawback of the plane was its poor towing speed which was barely more than 130 mph, less than a third of the speed of a US P-51 Mustang. As a result, it was kept away from the frontlines as much as possible.
High Hopes
Only twelve He-111Zs were ever produced and were used by the Luftwaffe to support its fleet of around 200 Me231 heavy gliders. The German Air Force had high hopes for this aircraft but all of this was dashed in 1944 when Hitler decided that all bomber production was deceased so that the factories could concentrate on producing more fighter aircraft. None of the Heinkel He-111 aircraft survived the war but the twin aircraft concept did live on.
When the Americans successfully developed a twin Mustang aircraft called the North American F-82, 262 of these radar-equipped aircraft were produced, and they were in service in the US Air Force from 1946 to 1953. The Me 321 was eventually developed into the Me 323 which had six engines eliminating the need for the tug aircraft to get them airborne.