In 1946, two B-17Gs were modified as flying testbeds for experimental turboprop engines. The Boeing company number Model 299-Z was assigned to these planes. The military equipment was removed, the pilot's cockpit was moved farther back, and the nose was completely modified to accommodate the experimental engine.
The first conversion was of B-17G-110-VE serial number 44-85813. It was turned over to the Wright Aeronautical Company under a bailment contract as EB-17G, the E prefix meaning that the aircraft was exempt from all but the most urgent technical orders issued for the type. The aircraft was fitted with a 5500 hp Wright XT35 Typhoon turboprop in the nose. This engine was more powerful than all four of the standard Wright Cyclone piston engines operating together. However, the Wright Typhoon was ultimately unsuccessful, and did not go into production. The aircraft was later used to test the Wright XJ65 turbojet, the engine being slung below a streamlined nose structure and the intake being covered with a cap for protection during ferrying.
The designation of this plane was changed to JB-17G in October of 1956, the J prefix having been introduced in 1955 to designate aircraft temporarily assigned to test work. In 1957, the plane was sold to Wright, which continued to use it as a five-engined testbed under the civil registration of N6694C. That year, it was used to test the R-3350 turbo-compound engine. The plane was later sold to an air tanker operator, and the missing nose was replaced by a hemispheric cap. N6694C crashed on takeoff in 1980 during a tanker mission and was damaged beyond repair. Its remains were purchased by warbird restorer Tom Reilly of Kissimmee, Florida for use in restorations of other B-17s.
The second conversion was of surplus B-17G-105-VE serial number 44-85734, which was sold to Pratt and Whitney for use in engine testbed work. It was converted at Seattle and was fitted with a dummy nose prior to delivery. It was assigned the civilian registry of NX-5111N. Following delivery to Pratt & Whitney at Hartford, Connecticut, an experimental XT-34 turboprop was fitted in the nose. The XT34 turboprop eventually went into production and ended up powering the Douglas C-133 Cargomaster long-range transport aircraft. A Pratt & Whitney T64 turboprop was installed briefly to test different engine and propeller combinations.
Following the completion of the tests in 1967, NX-5111N was donated to the Connecticut Aeronautical Historic Association based at Bradley International Airport. In 1979, it was heavily damaged in a tornado. In 1987, the damaged hulk was traded to Tom Reilly of Kissimmee, Florida, who plans to restore the aircraft to flying status in its original military configuration.
A third conversion was in the form of B-17G-110-VE serial number 44-85747, which was employed by Allison for use as a five-engine testbed. Unlike the first two, this conversion did not require that the cockpit be moved aft. It was retired prior to the establishment of JB-17 designation.
Other EB-17s included a number of SB-17Gs diverted to the Air Force Missile Test Center at Patrick AFB in Florida beginning in 1952. These planes were equipped with loudspeakers and VHF radios to warn boats and aircraft away from the area prior to missile test shots. They remained on duty at Patrick AFB until 1958. The EB-17 s became JB-17s in 1955, when the E prefix was replaced by the J and N prefix. The E prefix was reassigned to designate aircraft intended for the early-warning role.