Art Drawing of Ju635
The construction of socalled Tandem aircraft was a privilage area of Heinkel Flugzeugbau. The He111Z was the first tandem construction, which consisted of two Heinkel He111 aircraft, which were combined by common center wing section. This design was capable to take additional fuel for long range flights and was excellent for survey missions.
In 1944 the RLM asked Heinkel to create another tandem design from the Dornier Do 335, of which Heinkel had already developed the night fighter version. The project was developed at Heinkel under the designator P.1075 and was retransfered to Dornier for the serial production of the Do 635. But as the Do335 seemed not to be an adequate aircraft for long range survey missions, the design was transfered for reviews to Junkers. At Junkers the design was further developed as Ju 635 since autumn 1944. The fuselage was enlarged, additional fuel tanks were added under the fuselage, a pressured cabin for three crew members was added and the undercarriage was strengthened. The complete design was simplified for a more effective serial production. Nevertheless, most components of the Do335 could be used for the serial production of the Ju635. In November 1944 the Ju635 developement was stopped by RLM due to the fighter program. No prototypes were built.
Further Reading at other Sites:
Luft'46 (Dan Johnson, very detailled description, images + art)