The Boeing YB-40: how the B-17F strategic bomber was turned into a four-engined escort fighter - The Aviation Geek Club
VAR.B-40
B-17F-40 Flying Fortress
World War II US Gunships: YB-40 Flying Fortress and XB-41 Liberator Bomber Escorts (X-Planes): Wolf, William, Tooby, Adam: 9781472844699: Amazon.com: Books