"Pumpkin bombs" were conventional high explosive aerial bombs developed by the Manhattan Project and used by the United States Army Air Forces against Japan during World War II. The name "pumpkin bomb" resulted from the large ellipsoidal shape of the munition and was the actual reference term used in official documents. 486 of the bombs were built and 49 dropped on Japanese targets by the 509th Composite Group.
The concept for the pumpkin bomb originated with Navy Captain William S. Parsons of the Ordnance Division at Los Alamos and USAAF Colonel Paul W. Tibbets, commander of the 509th CG, on December 13, 1944, as a means of providing continued realistic training for the B-29 crews assigned to drop the atomic bomb after their deployment to the Western Pacific. The bomb would be a close but non-nuclear replication of the Fat Man plutonium bomb with the same ballistic and handling characteristics. Mission parameters would be similar to those of the actual atomic bomb missions and all targets located in the vicinity of the cities designated for atomic attack.
The development of the bomb was managed by the California Institute of Technology under the direction of Dr. Charles C. Lauritsen. Specifications for the bomb required that it be carried in the forward bomb bay of a Silverplate B-29 and be fuzed to be effective against actual targets. The bomb shells were manufactured by two Los Angeles, California, firms, Consolidated Steel Corporation and Western Pipe and Steel Company, while the tail assembly was produced by Centerline Company of Detroit, Michigan. After initial development, management of the program was turned over to the U.S. Navy Bureau of Ordnance in May 1945.
Pumpkin bombs were produced in both inert and high explosive variants, with the inert versions filled with a cement-plaster-sand mixture combined with water to the density of the Composition B used in the high explosive versions. The filler of both variants had the same weight and weight distribution as the inner sphere used in the plutonium bomb. All of the inert versions were shipped from the manufacturers directly to Wendover Army Air Field, Utah, by rail, where they were used by the 216th Base Unit in flight testing of the bomb shape.
The pumpkin bombs were externally similar to the Fat Man bomb in size and shape, and both had the same 52-inch square tail assembly and single-point attachment lug. The pumpkin bomb had three contact fuses arranged in an equilateral triangle around the nose of the bomb while the atomic bomb had four fuse housings. The atomic bomb had its sections bolted together but most if not all of the pumpkin bombs were welded with a four-inch hole used for filling the shell. The Fat Man also had four external mounting points for radar antennas which the pumpkin bombs did not have.
The bombs intended as live ordnance were shipped to the Naval Ammunition Depot, McAlester, Oklahoma, for filling with explosives. The Composition B was poured as a slurry, solidified in a drying facility, sealed, and shipped by railroad to the Port Chicago Naval Magazine, California for shipment by sea to Tinian.
The pumpkin bombs were twelve feet eight inches in length and five feet in maximum diameter. The most commonly-given weight for the bombs is 5.26 tons, consisting of 3,800 pounds for the shell, 425 pounds for the tail assembly, and 6,300 pounds of filler. The shells were made of .375-inch steel plate and the tail assemblies from .200-inch aluminum plate. Although anecdotal sources attribute the name of the bombs to painting of them pumpkin color, there is no evidence that the bombs were ever painted other than the standard olive drab used by the Army Air Forces on all live ordnance. (Campbell, The Silverplate Bombers, "Chapter Notes", Chapter 7 note 5, p. 220)
In addition to the live ordnance dropped on Japan, several hundred inert bombs were used by combat crews of the 509th's 393rd Bomb Squadron to test the ballistics of the bomb shape in conjunction with Project Alberta, and to train bombardiers in mission procedures before movement of the group overseas.