The E4/20 included numerous innovations, including its all-metal monocoque construction, onboard facilities such as lavatory, kitchen and radio communications,
as well as its notable and sturdy monoplane load-bearing box-girder wing constructed of dural metal which formed both the wing's main girder and the structure of the wing itself.
Skinned with thin sheets of dural metal to give the aerofoil shape necessary for a wing, the girder section wing had fabric covered leading and trailing edges attached to it.
This superb and innovative wing was robust and self-supporting.
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Initial shape was open cockpit. Later fully enclosed pilot's cabin with Cellon windows was installed.
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