Thought you might find this of interest....Douglas XB-19
The purpose of the XB-19 project was to test the flight characteristics
and design techniques associated with giant bombers. Douglas Aircraft Company strongly wanted
to cancel the project, because it was extremely expensive. Despite advances in technology
that made the XB-19 obsolete before it was even completed, the Army Air Corps felt that the prototype
would be useful for testing. Its construction took so long that competition for the contracts
to make the Northrop XB-35 and Convair XB-36 occurred two months before its first flight.
The plane finally flew on June 27, 1941, more than three years after the construction contract was awarded.
In 1943, the original Wright R-3350 engines were replaced with Allison V-3420-11 V engines.
After completion of testing, the XB-19 served as a cargo carrier until it was scrapped in 1949.
here's a link;http://en.wikipedia.org/wiki/Douglas_XB-19