XB-28 to XB-43
The next twin-engine bomber to appear had been designed originally in August 1939 to a specification for high-altitude versions of the B-25 and B-26. Both the Martin XB-27 (Model 182) and North American XB-28 (NA-63) were to have turbosupercharged Pratt & Whitney Wasps and pressurized cabins. The Martin project was to have a wingspan increased to 84 feet, but was never built.
Two XB-28s, however, were ordered on February 13, 1940, and the first was flown on April 26, 1942. Powered by the R-2800-11, the XB-28 had tricycle landing gear, single rudder, five men in a pressure cabin, and carried 4,000 pounds of bombs and six .50-caliber guns with 1,200 rounds. The guns were paired in upper, lower, and tail turrets remote-controlled by pairs of periscope sights above and below the forward cabin. The second prototype was completed as the XB-28A photo version with R-2800-27s.
Although the high-altitude performance of the XB-28 far exceeded that of service types, wartime medium bombing was done at relatively low altitudes and authorities were unwilling to interrupt Mitchell production for an untried type. Instead, the main emphasis of bomber design in 1940-1942, from the B-29 to the B-39, would be on large long-range types.
In response to a new medium bomber specification issued October 15, 1940, Martin submitted Model 189 with two turbosupercharged R-3350s, and an Air Force Letter of Intent on February 11, 1941, designated the project XB-33. But on April 28, the design was changed to the Model 190 with four turbosupercharged 1,750-hp R-2600-15 Wrights and twin tails. Two XB-33 prototypes were ordered June 11, the mockup was approved on October 6, and shortly after Pearl Harbor a Letter of Intent for 402 B-33As to replace the B-26C at the Omaha factory was sent on December 24, and a contract approved January 17, 1942.
The high-wing XB-33 “Super Marauder” was to have a 134-foot span, 95,000 pound gross weight, and carry seven men with a three to six-ton bomb load. Top speed was to be 345 mph at 35,000 feet and range 1,930 miles with 12,000-pounds of bombs. Eight remote-control .50-caliber guns, four in the top turret, two in the tail, and two in the belly, were to arm the prototype, while the B-33A was planned with ten guns paired in nose, top, tail and two belly turrets.
But longer range was demanded and on November 9, 1942, General Oliver P. Echols proposed replacing the B-33 with a heavier aircraft like Northrop’s B-35 flying wing design. Instead, all B-33 work was stopped on November 25 to make way at Omaha for the B-29, which had already demonstrated superior range.
A fitting end to the story of bomber designs with two propellers was the Douglas Model 459 begun in May 1943 and proposed to the AAF on June 15, 1943. The original design was designated XA-42 with two Allison V-1710-93 liquid-cooled engines in the fuselage. When the experimental contract was awarded on June 25, however, it was seen as a way of “increasing the heavy long-range bombing attack with minimum industrial effort.”
Redesignated XB-42 on November 25, 1943, it became a high-speed bomber that could match the B-29s striking range at much less cost in economic strain, fuel, and crew requirements. First flown on May 6, 1944, the XB-42 minimized drag by burying both inline engines in the fuselage behind the pilots and extending drive shafts (each of five P-39 shafts) to co-axial Curtiss pusher propellers behind the tail. Radiators in the wings cooled 1,800-hp Allison V-1710-103s.
The airplane as a scout had developed into the airplane as a bomber and as a destroyer of other aircraft, but this development was European, not American, and happened without comparable activity on this side of the Atlantic. New fighters often made sensational appearances over the front and disappeared from battle in only a few months. America missed the rapid technological advances that produced five generations of fighters in four years.
The usual bombardier sat in the transparent nose, with the pilot and co-pilot seated under individual canopies, the latter facing backward when operating the remote-controlled guns. Two .50-caliber guns in the trailing edge of each wing were limited to a 50 degree horizontal and 45 degree vertical swing, but were thought enough to meet attacks that, because of the bomber’s speed, could come only from the rear. Two fixed forward guns were located in the nose, and a total of 2,400 rounds were provided for all six guns. An alternate attack solid nose with eight guns was anticipated. A range of over 5,000 miles with a one-ton bomb load was expected, but the bomb bay had an 8,000-pound capacity, and normal range was 1,840 miles.
A second XB-42 flown on August 1, 1944, was later given a more conventional canopy, and was lost on December 16, 1945, shortly after setting a 433-mph transcontinental flight record, with the help of tail winds. When jet engines became available, the design evolved into the XB-43 described in a later chapter. Douglas did propose the addition of a 1,600-pound static thrust Westinghouse jet under each wing on February 23, 1945, so the first prototype was reworked in this manner and flown again as the XB-42A on May 27, 1947. All guns were removed, but fuel capacity was increased.
Although none of the experimental medium bombers designed after 1939 went into production, more than enough of earlier models to win the war were available. From 1940-1945, over 30,000 four-engine heavy and over 15,000 twin-engine medium bombers had been built. Germany was defeated and the Japanese Empire had been pushed back, but the vast distances across the Pacific protected Japan from deadly attacks by those types. Closing these distances required a naval offensive and the massing of very long-range giant bombers, so these aircraft developments are described in later chapters.