The Boeing B-29 Superfortress is a WWII era long range, strategic heavy bomber that was produced in many experimental and production models. XB-29 Section source: Baugher The XB-29, Boeing Model 345, was the first accepted prototype or experimental model delivered to the Army Air Corps, incorporating a number of improvements on the design originally submitted, including more and larger guns and self-sealing fuel tanks. Two aircraft were ordered in August 1940, and a third was ordered in December. A mockup was completed in the spring of 1941, and it first flew on September 21, 1942. Testing was conducted on the XB-29 until February 18, 1943, when the second prototype crashed. The flight was conducted by Boeing's chief test pilot, Edmund T. "Eddie" Allen on a two-hour powerplant performance test. The accident happened when leaking fuel from a filler cap in the wing leading edge ran down inside the leading-edge and ignited. The fire spread to the engines, and due to the much reduced power, the aircraft, unable to climb, crashed into the Frye meat-packing plant. The crash demolished the majority of the packing plant and killed all eleven XB-29 crew, 22 employees at the plant, and one fireman. Many elite Boeing personnel involved in the design perished in the accident; the pilot, Allen, was chief of the Research Division. After the crash, the United States Army Air Forces and a congressional committee headed by then-Senator Harry S. Truman investigated the B-29 program, issuing a scathing report, prompting the Army Air Forces to take control of the program. YB-29 The YB-29 was an improved XB-29 and 14 were built for service testing. Testing began in the summer of 1943, and dozens of modifications were made to the planes. The engines were upgraded from Wright R-3350-13s to R-3350-21s. Where the XB-29 had three-bladed props, the YB-29 had four-bladed propellers. Various alternatives to the remote-controlled defensive systems were tested on a number of them, particularly the fourth one delivered. After alternative arrangements had been fully tested, defensive armament was standardised at ten .50-calibre machine guns in turret-mounted pairs. The YB-29 also featured a better fire control system. B-29 The B-29 was the original production version of the Superfortress. Since the new bomber was urgently needed, the production design was developed in tandem with the service testing. In fact, the first B-29 was completed only two months after the delivery of the first YB-29.