Mantelli – Brown – Kittel – Graf 
The Heinkel He 177 [EPUB ebook] 

The Heinkel He 177 ‘Greif’ (Griffin in German) was a heavy bomber force in the Luftwaffe during World War II.
The Greif was the only heavy bomber built in appreciable quantities from Germany.
The motors mounted in pairs were difficult to cool, and this reduced the reliability of the machine for a long time. The real solution was found in the He 277, with 4 separate engine nacelles, as well as in the He 274, for stratospheric bombing; as often happened, however, it was too late for the outcome of the war.
Designed to meet a requirement of 1936, known as Bomber A, the aircraft was originally intended to be a purely strategic bomber with the task of supporting a bombing campaign in the long run against the Soviet industry in the Urals.
The Greif had four engines mounted in pairs in tandem so as to appear outside a twin-engine; this arrangement that the Heinkel and its designer had created, had its explanation in the grounds to oppose lower air resistance than traditional 4 engines installed on the leading edge of the wing.
The problem, which materialized immediately was overheating of the engines as the plane was flying.
This is because the arrangement in tandem did not favor a good cooling of the engine mounted behind than in front despite the installation of radiators on the leading edge.