The XB-15 was delivered to the 2nd Bombardment Group at Langley Field, Virginia and was pressed into humanitarian service in 1939 following an earthquake in Chile. The aircraft then underwent official bombing tests in Panama beginning in April. During the tests, accuracy proved elusive for less than 1% of dropped ordnance actually hit its mark. Several load-to-altitude records were then set by XB-15 crews in the period following - on July 30th, 1939, the XB-15 recorded a flight carrying a 31,205lb load up to an altitude of 8,200 feet. Officially (according to Boeing sources), the XB-15 was cleared for a combat ordnance load of up to 8,000lbs. Her final militarized form also included a defensive network of 3 x 0.30 caliber machine guns and 3 x 0.50 caliber heavy machine guns including one at her glazed-over nose. In 1940, the machine guns were removed when the aircraft arrived at Duncan Field, Florida.
With the United States entry into the war following the Japanese surprise attack on Pearl Harbor in December of 1941, the XB-15's future became a limited one as technological advancements overtook her once-evolutionary status. The B-17 was firmly entrenched at a primary bomber for the USAAF (as was the Consolidated B-24 Liberator) while the B-29 was nearing operational status - the latter soon to become its own technological marvel by war's end.
It was decided that the existing XB-15 airframe be converted as a dedicated cargo transport and, on May 6th, 1943, the aircraft was redesignated under the experimental "XC-105" classification. The fuselage was given a pair of cargo doors and an internal hoist system was installed to what would make the XC-105 become the air force equivalent of a pack mule. The aircraft served in this role until December 18th, 1944 before seeing her official retirement by the USAAF (United States Army Air Forces). With the end of the war in Europe by May 1945, the airframe of XB-15/XC-105 was ordered stripped and scrapped while based at Albrook Field in Panama. Her shell was dropped in a swamp near the airfield where she sank. During her active tenure, the XB-15 only served two US groups - the aforementioned 2nd Bombardment Group of the USAAC and the 20th Troop Carrier Squadron of the USAAF.
Work on the XB-15 went on to influence the proposed private venture Boeing Y1B-20 heavy bomber design of which two were ordered by the US Army in 1938. However, this order was quickly cancelled before physical work had begun. Nonetheless, Boeing's work on such heavy-class military bomber aircraft provided the know-how in the design, development and production of several key operational "heavies" for United States air power during World War 2 and beyond - even leading up to Boeing's involvement in commercial passenger flight.
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