Langham Dome in north Norfolk, one of only six remaining training domes in the country, was built in 1942 and sits on the edge of a former RAF base.
Film of enemy planes was projected onto its walls for target practise.
A museum in the restored concrete structure is now open to visitors following grants from English Heritage and the Heritage Lottery Fund.
Listed as a Scheduled Ancient Monument in 1986, plans to restore the dome at the former RAF Langham, seen at the forefront of both defence and attack during WW2, began more than 20 years go.
The restoration of the 25ft (7.6m) tall and 40ft (12m) wide dome got under way in 2013 after the project secured grants of nearly Â£650,000.
One of the key issues was to protect the metal grid framework on which the dome is based, said the Friends of Langham Dome.
Extensive work, involving 17 tonnes of rendering sand on the shell, was also required to ensure it was watertight.
Malcolm Crowder, from the North Norfolk Historic Buildings Trust, said: "The dome was built essentially as a temporary structure over 70 years ago.
"Crumbling concrete on the outer surface had allowed water to get in, causing serious deterioration to the metal framework. This became a vicious circle of decay for both the concrete and metal structure.
"This situation has been reversed and the building... is now fit to grace the Norfolk skyline and educate the public for at least another 50 years. It's a truly outstanding heritage project achievement."
The Langham Dome Museum is open until the end of October.
Restoration on an at-risk World War Two building which was used to train anti-aircraft gunners has been completed.