Model 145B XB-16B XLRB-4
In 1935, in response to the Project D requirement, Martin revised the four-engine low-wing XB-16 design to the six-engine high-wing Model 145B, making the aircraft much, much bigger [the wing area nearly doubled]. Indeed, the two aircraft had little in common apart from similar designations. Curiously, the reported performance of the enlarged design was signicantly reduced from that of the initial design, a fact that goes unmentioned in standard reporting. Possibly this was a product of greater design realism, or possibly a product of the jumble of data on this rather poorly attested program. The record is further clouded by some uncertainty as to whether the design was an Army product given to Martin for futher elaboration, or a Martin design the rights to which were sold to the Army [the later seems more probable].
The seemingly contemporaneous artwork [of unattributed provenance] of a six engine bomber that is universally associated with the B-16 resembles neither the initial nor the final design. The designs reported in the authoritative Lloyd S. Jones's book U.S. Bombers (Aero, 1974) seem plodding and unimaginative compared to the sleek and racy design of the artwork.
Initially, the Model 145A XB-16 design had been roughly equivalent in dimension to the Boeing submission but Martin increased the Model 145B B-16's size to meet the bomb load and range requirements (an increase to internal space allowed for greater fuel loads to be carried and lifting and strength properties could be spread out over the larger aircraft as a result).
In order to increase the range and bomb load, the wing span was increased from 140 ft (43 m) to 173 ft (53 m) - even greater than the World War 2-era Boeing B-29 "Superfortress" still to come. The larger aircraft required more power so an additional pair of V-1710 engines was added. Along with the original set of four on the leading edge of the wing (tractors), the two additional engines were positioned to face aft at each wing trailing edge (pushers). In all each wing would mount three engines apiece with the new engine installs added directly behind the existing outboard fits. Between the two nacelles was a large hull gondola, in which the cabin of the bomber crew was placed.
The tail booms emanated from the wing trailing edges and were capped at their absolute ends by vertical tail fins. The elevation consisted of a large horizontal stabilizer and two vertical stabilizers mounted at its ends. The booms were joined to one another by a shared horizontal plane that also protruded from the vertical fin sides outboard.
The bomber's chassis was concealed, three-support with a front support. The main chassis had double wheels. Lloyd S. Jones's book U.S. Bombers (Aero, 1974) reported "It is interesting to note the first appearance of the tricycle type landing gear on this projected design. The large diameter main tires were partially exposed when the gear retracted into the wing. Estimated landing speed was 77 mph."
This version had an estimated range of 3,300 miles carrying 2,500 pounds of bombs and a crew of 11 men. This version had a wingspan 20% greater than that of the B-29 Superfortress, the first operational bomber that would fill the role intended for the XB-16. It was considered that such an avant-garde design would be too expensive and difficult to operate, and allison engines used in the machine did not provide the construction of the required power at high altitudes. The XB-16 was considered as being too large and expensive, and the project was cancelled before anything could be built.
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