History
In 1943, the US Navy issued a requirement for a smaller dive bomber design intended for escort carrier operations. Many American escort carriers lacked the capacity to house large dive bombers, which were typically stored alongside torpedo bombers and fighters with folding wings. As a result, the Navy sought lighter and slimmer dive bombers to complement its growing escort carrier fleet.
The specifications outlined a single-seat, single-engine, carrier-based dive bomber powered by a Pratt & Whitney R-2800-22W radial piston engine, driving a four-blade propeller. The armament was to include two wing-mounted cannons, and to keep the aircraft compact, all three stores would be mounted externally and rated for up to 1,000 lbs. A key feature was the ability to fold the wings for improved storage aboard escort carriers.
Kaiser-Fleetwings responded to the Navy’s 1943 requirement by submitting a proposal in January 1944. This company was an oddball in military aircraft production, as it primarily built trainers and other light civilian aircraft with no prior experience in combat aircraft design.
The Navy approved the design and awarded Kaiser-Fleetwings an initial contract to build two XBK-1 prototypes powered by R-2800-22W engines on February 23, 1944. In March 1944, the contract was amended to increase the number of aircraft to five.
By fall 1944, Kaiser-Fleetwings presented a mockup for review, and the Navy awarded a pending contract for 17 pre-production units in October. However, the Navy decided to categorize the Kaiser-Fleetwings XBK as a bomber-torpedo aircraft, leading the company to revise the design to accommodate a 2,000-lb MK 13-2 torpedo. As a result, the XBK-1 was redesignated XBTK-1.
The initial configuration of the XBTK-1 featured a forward-mounted engine with a forward-set cockpit. Its low-mounted monoplane wings were equipped with two 20 mm cannons and eight rocket mounts on the folding wing panels. Additional features included self-sealing fuel tanks, 230 pounds of cockpit armor, and landing flaps with a split “picket fence” dive brake. “Finger” brakes were positioned on the upper wing surface ahead of the flaps. To prevent empennage buffeting when the dive brakes were deployed, the horizontal tail was mounted on the tailfin above the fuselage. The aircraft was also designed to accommodate an AN/APS-4 radar, with fuel stored in a single fuselage tank. One of the XBTK-1’s most distinctive features was its exhaust system, which directed engine exhaust gases through ducts in the forward fuselage aft of the cockpit. This system was designed to reduce cooling drag and generate additional thrust, a concept known as the exhaust pump.
As the XBTK-1 project faced delays and fell six months behind schedule, the first prototype completed its maiden flight on April 12, 1945. Flight testing revealed inadequate engine cooling and severe fuselage vibrations caused by the exhaust pump. To address these issues, the aircraft was returned to the Fleetwings engineers in May 1945 for modifications. The original engine was replaced with a more powerful 2,100-hp supercharged R-2880-34W. The Navy recommended replacing the unique exhaust system, as it caused excessive cockpit heat and made entry and exit difficult after engine shutdown. As a result, conventional exhaust with cowl flaps replaced the exhaust pump to improve cooling. The Navy received the modified XBTK-1 prototype for service testing around the end of World War II. It was not until July 1945 that the aircraft underwent flight trials, reaching a maximum speed of 373 mph. In August 1945, the prototype was delivered to the US Navy Naval Air Test Center at Naval Air Station Patuxent River, Maryland, for further evaluation.
The second production aircraft first flew in March 1946, featuring leading-edge slots that improved stall performance. It went on to demonstrate satisfactory stall characteristics, including a lowered stall speed. However, the spin-recovery performance resulting from a stall was poor, particularly with the power off. However, the aircraft’s diving characteristics were superior to any dive bomber in service or under development.
Following World War II, the Navy reduced its order from 20 to 10 aircraft. In May 1946, the decision was made to complete only five airframes already in production, and the program was officially terminated in September 1946, as the role had already been filled by the Douglas AD Skyraider and Martin AM Mauler. Of the five completed XBTK-1s, only four were able to fly, and these aircraft were eventually scrapped. The XBTK-1 program was ultimately canceled due to the end of World War II and the emergence of more capable attack aircraft.
