Martin Model 145 B-16
In 1933, the US Army Air Service Material Division at Wright Field began a study for a bomber aircraft called the hemisphere defender. The specifications called for a long range bomber capable of the destruction of distant land or naval targets by bombs, and the ability to reinforce Hawaii, Panama and Alaska without the use of intermediate servicing facilities. The project is known under the designation Project A. This modern bomber that would able to carry 2500-lb / 900 kg bomb load for 5000 miles at a speed of 200mph / 320 km/h. This specification was submitted to the War Department as "Project-A" and received tentative approval. In February 1934, the General Staff of the War Department allocated a sum of US$609,300 for the program.
On April 14, 1934, the United States Army Air Service announced a design competition for the new heavy bomber. On 12 May 12, 1934, negotiations were started with two companies whose projects were submitted to the competition. Boeing, which offered its construction marked as Boeing XB-15 (Model 294) and Martin, which designed the plane originally designated as Model 145. The Martin Model 145 was initially similar to Boeing's XB-15; both were designed for four Allison V-1710 liquid-cooled engines. The Martin XB-16 was designed as a long range bomber capable of carrying a 2,500-pound bomb load 3,300 miles [5,000 km]. The aircraft would have had a wing span of about 140 feet and an estimated top speed of 235 mph.
To respond to the requested range and performance, a four-engined layout was selected. The aircraft was similar in size to the Boeing XB-15, but was to use four 1,000 hp Allison V-1710 liquid-cooled inline engines instead of air-cooled radial engines normally used on bombers of the 1930s.
The liquid-cooled inline piston engines was a departure for the Army as contemporary large aircraft relied on air-cooled radial piston engines. Liquid-cooled inline engines aided high-altitude performance and could be fitted within more streamlined nacelles, therefore inherently improving aerodynamic efficiency. But the trade off in selecting an inline engine, was in the increased vulnerability to enemy fire due to the internally fragile nature of such an engine. The Allison liquid cooled engines were buried entirely in the wings driving 12 foot 3 inch propellers via extension shafts.
The wide-spanning wings with a 140 foot span were high-mounted. Each wing was to hold a pair of engine nacelles that were well-contoured into the general shape of the wing elements themselves. The cockpit was stepped (the pilot's overlooking the aircraft's nose section) and window panes accompanied the bombardier's position at the nose.
The inboard nacelles were enlarged to accomodate the dual five foot tires of the landing gear. The standard operating crew would number ten.
Lloyd S. Jones's book U.S. Bombers (Aero, 1974) reported "Estimated top speed, with supercharging, at 20,000 feet was 237 mph. Service ceiling was placed at 22,500 feet with a rate of climb of 740 feet per minute. A landing speed of 60 mph was anticipated by the use of the Fowler type flaps of 360 square feet which spanned 80 feet. Defensive armament was to be carried in plexiglas nose and tail enclosures and retractable dorsal and ventral turrets. A bomb load of 12,180 pounds was to be transported 3,200 miles, and flight endurance with 4,238 gallons of fuel was 42 hours. A crew of ten was to have operated the aircraft."
Martin's Model 145 was accepted to build a XB-16 prototype. On June 29, 1934, the United States Army Air Corps signed a preliminary agreement with Boeing for the construction of a bomber prototype labeled XBLR-1 (in July 1936, the designation was changed to XB-15), but still sustained interest in Martin's construction in the event that the Boeing project failed. Some sources suggest that the Martin 145A was to have been the XBLR-4 but there doesn't seem to be much evidence to support this claim.
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