The pilot reported that the airplane departed with 122 gallons of fuel for the cross-country flight from Destin, Florida, to Quincy, Illinois. The airplane's two inboard wing fuel tanks, each having a 36 gallon capacity, were completely full before departure; however, in order to keep the airplane below its maximum gross weight at departure, the two outboard wing fuel tanks were each partially fueled with 25 gallons. The pilot reported that the inboard fuel tanks were used for engine startup, taxi, takeoff, and climb to the initial cruise altitude assigned by air traffic control. After reaching the initial cruise altitude, he switched to the outboard fuel tanks and leaned the mixture setting for optimal fuel consumption. About 2 hours into the flight, the pilot switched back to the inboard fuel tanks after the engines had consumed the useable fuel in the two outboard fuel tanks. The pilot reported that when the flight was about 7 miles from the destination airport, descending through 3,500 feet mean sea level, the right engine lost total power. The pilot feathered the right propeller, secured the right engine, and continued toward the planned destination. Shortly thereafter, the left engine lost total power and the pilot feathered the propeller and secured the engine. The pilot reported that he delayed extending the landing gear until the airplane cleared a row of trees that bordered the harvested soybean field that he selected for the forced landing. After clearing the trees, the pilot attempted to extend the landing gear by moving the landing gear selector handle to the down position; however, because the left engine was inoperative, a normal landing gear extension with the engine driven hydraulic pump was not possible. The pilot immediately attempted to extend the landing gear using the emergency hand-pump, but the airplane impacted terrain before the landing gear was fully extended. Upon contact with the terrain, the nose and left main landing gear collapsed. The left wing and fuselage sustained substantial damage during the forced landing.A postaccident examination of the airplane established that the inboard and outboard fuel tanks were void of any useable fuel. The pilot reported that the accident flight was about 4 hours 20 minutes in duration and that fuel exhaustion was the reason that both engines had lost total power. Before departing on the accident flight, he estimated that the flight would require about 118 gallons of fuel, and as such, there would be about 4 gallons of fuel remaining at the completion of the flight. However, after the accident, the pilot acknowledged that he failed to account for the 6.8 gallons of unusable fuel within the fuel system. Additionally, the airplane was equipped with an emergency carbon-dioxide blowdown system that the pilot could have used to rapidly extend the landing gear instead of the more laborious and time consuming emergency hand-pump. The pilot did not offer an explanation on why he chose to use the emergency hand-pump instead of the carbon-dioxide blowdown system.