Taken from "Big Bombers of WWII" (ISBN 0-681-07570-8)....
"The dissatisfaction with cramped quarters in the noses of turret-equipped Liberators led to one of WWII's most bizarre surgeries. The complete forward fuselage of a B-17G Flying Fortress was grafted to a B-24J. According to an AAF test summary, the massive splice job resulted in an operationally unsuitable airplane."
Scan down a bit in the paragraph...
"The findings: The operational performance of this aircraft is poor in all respects... lacked directional and longitudinal stability, especially at altitude... disappointing service ceiling of only 18,000 feet."