The Douglas XB-7: After starting out as a promising design, the Douglas XB-7 (originally the XO-36) became obsolete as its features were surpassed by those of the Martin B-10. This gull-winged twin-engine bomber prototype had corrugated metal skin and engine nacelles that were held away from both the wing and fuselage by struts. It had retractable landing gear. The type was not put into production.
