TYPE: Medium turbojet-driven bomber

ACCOMMODATION: Crew of three

POWER PLANT:  Six General Electric J35 turbojet engines, rated at 1.734 kp each

PERFORMANCE: 523 mph at 35,000 ft

COMMENT: The Martin XB-48 was an American medium jet bomber developed in the mid-1940s. It competed with the Boeing B-47 “Stratojet”, which proved to be a superior design, and was largely considered as a backup plan in case the B-47 ran into development problems. It never saw production or active duty, and only two prototypes were built.
In 1944, the U.S. War Department was aware of aviation advances in Germany and issued a requirement for a range of designs for medium bombers weighing from 80,000 lb (36,287 kg) to more than 200,000 lb (90,718 kg). Other designs resulting from this competition, sometimes named “the class of ’45”, included the North American XB-45 and the Convair XB-46. Production orders finally went to the North American B-45 “Tornado”, and even this airplane served only for a couple of years before again being replaced by the much more modern Boeing B-47 “Stratojet”, although the B-45 had enough “utility” built in to maintain a niche as a reconnaissance aircraft.
In retrospect, the “class of ’45” were transitional aircraft, combining the power of turbojets with the aeronautical knowledge of World War II. The XB-48 was no exception, as its round fuselage and unswept wings showed a distinct influence of Martin’s B-26 “Marauder” medium bomber. Still, where the B-26 had enough thrust with two massive 18-cylinder radial engines, the XB-48 needed no less than six of the new jet engines.
Although the pictures make it look as if the aircraft had three engine nacelles under each wing, the jet engines were actually clustered in a pair of flat three-engined nacelles with an intricate system of air ducts between the engines, intended to facilitate cooling. At the time of the XB-48’s design, jet propulsion was still in its infancy.
The XB-48 was the first aircraft designed with bicycle-type tandem landing gear, which had previously been tested on a modified B-26. The wing airfoil was too thin to house conventional landing gear mechanisms. The main landing gear was in the fuselage and small outriggers located on each wing were used to balance the aircraft.
The XB-48 made its first flight on 22 June 1947, a 37-minute, 117 km hop from Martin’s Baltimore, Maryland plant to NAS Patuxent River, Maryland, but blew all four tires on its fore-and-aft mounted undercarriage on landing when the test pilot applied heavy pressure to the specially-designed, but very slow to respond, insensitive air-braking lever.
In 1948 the Martin XB-48 program was cancelled (Ref.: 24).

TYPE:  Medium Bomber

ACCOMMODATION: Crew of five

POWER PLANT: Two Pratt & Whitney R-2800-27 air-cooled radial turbocharged engines, rated at 2,000 hp each

PERFORMANCE: 372 m.p.h. at 25,000 ft

COMMENT: The North American XB-28 (NA-63) Dragon was an aircraft proposed by North American Aviation to fill a strong need in the United States Army Air Corps for a high-altitude medium bomber. It never entered production, with only two prototypes being built.
The order for a high-altitude medium bomber was put out on 13 February 1940; the XB-28 first flew on 26 April 1942. The XB-28 was based on North American Aviation‘s’s highly successful B-25 Mitchell, but as it evolved it became a completely new design, much more reminiscent of the Martin B-26 Marauder. The overall configuration of the B-25 and XB-28 were fairly similar; the most important distinction was that the twin tail of the B-25 was changed to a single tail on the XB-28. It was among the first combat aircraft with a pressurized cabin.
The XB-28 proved an excellent design, with significantly better performance than that of the B-25, but it was never put into production. High-altitude bombing was hampered significantly by factors such as clouds and wind, which were frequent occurrences in the Pacific. At the same time, medium bombers were becoming much more effective at lower altitudes. The gains in aircraft performance that came with high-altitude flight were not considered sufficient to justify switching from low-altitude bombing.
Even though the Army Air Forces rejected the XB-28 as a bomber, they ordered another prototype. Designated XB-28A, it was meant to explore the possibility of use as a reconnaissance aircraft. The XB-28A crashed into the Pacific Ocean off Southern California after the crew bailed out on 4 August 1943 (Ref.: 24).

TYPE: High-speed medium bomber

ACCOMMODATION: Crew of three

POWER PLANT: Two General Electric J35-GE-3 turbojet engines, rated at 1,835 kp each

PERFORMANCE: 507 mph

COMMENT: USAAF leaders in the Air Material Command began to consider the possibilities of jet-propelled bombers as far back as October 1943. At that time, Douglas Aircraft was just beginning to design a promising twin-engine bomber designated the XB-42 “Mixmaster”. Reciprocating engines powered this aircraft but they were buried in the fuselage, leaving the laminar flow-airfoil wing clean of any drag-inducing pylon mounts or engine cowlings. The airframe appeared ideally suited to test turbojet propulsion. Douglas confirmed the feasibility of the concept and the USAAF amended the XB-42 contract in March 1944 to include the development of two turbojet-powered XB-43 prototypes, reduced from an initial order of 13 test aircraft. The Douglas design team convinced the Army that modifying the XB-42 static test airframe into the first XB-43 was a relatively straightforward process that would save time and money compared to developing a brand new design. Douglas replaced the two Allison V-1710 engines with a pair of General Electric J35 turbojets, then cut two air intakes into each side of the fuselage, aft of the pressurized cockpit. Removing the propellers and drive shafts freed enough space for two long jet exhaust ducts. Without any propellers present, there was no chance of striking the blade tips on the runway, so the entire ventral fin/rudder unit of the earlier XB-42’s full four-surface cruciform tail was omitted. Douglas compensated for the loss of yaw stability by enlarging the dorsal fin/rudder unit. The end of World War II caused a general slowdown within the aviation industry and General Electric was late delivering the engines. So America’s first turbojet bomber finally flew for the first time on 17 May 1946. Douglas Aircraft was keen to mass-produce the new bomber and the USAAF considered ordering 50, but these plans never became realized. The USAAF was already moving ahead with a new bomber, the North American XB-45 “Tornado”, designed from the outset for turbojet power and promising a quantum leap in every category of performance (Ref.: 24).

TYPE: High-speed medium bomber

ACCOMMODATION: Crew of three

POWER PLANT: Two Allison V-1710-125 liquid-cooled engine, rated at 1,325 hp each, driving three-bladed, contra-rotating propellers

PERFORMANCE: 410 mph at 23,440 ft

COMMENT: The XB-42 was developed initially as a private venture; an unsolicited proposal was presented to the USAAF in May 1943. This resulted in a contract for two prototypes and one static test airframe, the USAAF seeing an intriguing possibility of finding a bomber capable of the Boeing B-29 “Superfortress” range without its size or cost. The aircraft mounted a pair of Allison V-1710-125 liquid-cooled V-12 engines behind the crew’s cabin, each driving one of the twin propellers. Air intakes were in the wing leading edge. The landing gear was tricycle and a full, four surface cruciform tail was fitted, whose ventral fin/rudder unit prevented the coaxial propellers from striking the ground. The pilot and co-pilot sat under twin bubble canopies, and the bombardier sat in the extreme front behind a plexiglass nose. Defensive armament was two 0.50 in machine guns each side in the trailing edge of the wing, which retracted into the wing when not in use. These guns were aimed by the copilot through a sighting station at the rear of his cockpit. The guns had a limited field of fire and could only cover the rear, but with the aircraft’s high speed it was thought unlikely that intercepting fighters would be attacking from any other angle. The first XB-42 was delivered to the Army Air Force and flew at on 6 May 1944. Performance was excellent, being basically as described in the original proposal: as fast or even faster than the de Havilland “Moquito” but with defensive armament and twice the bomb-load. The end of World War II allowed the Air Force to consider possibilities with a little more leisure. Although with the second prototype additional Westinghouse 19XB-2A jet engines were mounted under the wings to enhance performance (XB-42A) it was decided to wait for the development of better jet bombers rather than continue with the XB-42 program (Ref.: 24).