Douglas B-18 Bolo

The Douglas B-18 Bolo is an American twin-engined medium bomber which served with the United States Army Air Corps and the Royal Canadian Air Force (as the Digby) during the late 1930s and early 1940s. The Bolo was developed by the Douglas Aircraft Company from their DC-2 as a replacement for the Martin B-10.

By 1940 standards, it was slow, had an inadequate defensive armament, and carried too small a bomb load. By 1942, surviving B-18s were relegated to antisubmarine, training and transport duties. A B-18 was one of the first USAAF aircraft to sink a German U-boat, U-654 on 22 August 1942 in the Caribbean.

In 1934, the United States Army Air Corps requested for a twin-engine bomber with double the bomb load and range of the Martin B-10 then entering service. During the evaluation at Wright Field the following year, Douglas offered its DB-1. It was competing against the Boeing Model 299 (later developed into the Boeing B-17 Flying Fortress) and Martin 146.

While the Boeing design was clearly superior, the 299's four engines eliminated it from consideration despite being the favorite, and the crash of the prototype — caused by taking off with the controls still locked — put its purchase on hold. The Martin 146 was a minor improvement on the B-10, and was never seriously considered. During the depths of the Great Depression, the lower price of the DB-1 at $58,500 compared to $99,620 for the Model 299 also favored the Douglas entry, and it was ordered into immediate production in January 1936 as the B-18.

The DB-1 design was modified from that of the DC-2. The wingspan was 4.5 ft (1.4 m) greater, the fuselage was narrower and deeper, and the wings were moved up to a mid-wing position to allow space under the spars for an enclosed bomb bay. Added armament included manually operated nose, dorsal, and ventral gun turrets.

At one point, Preston Tucker's firm received a contract to supply Tucker remote controlled gun turrets but these were unsuccessful, and were never used in service.

The initial contract called for 133 B-18s (including the prototype), using Wright R-1820 radial engines. The last B-18 of the run, designated DB-2 by the company, had a power-operated nose turret in a redesigned nose but this did not become standard. Additional contracts in 1937 (177 aircraft) and 1938 (40 aircraft) were for the B-18A, which had the bombardier's position further forward over the nose-gunner's station in a wedge shaped nose and the B-18A was fitted with more powerful engines.

Deliveries of B-18s to Army units began in the first half of 1937, with the first examples being test and evaluation aircraft being turned over to the Materiel Division at Wright Field, Ohio, the Technical Training Command at Chanute Field, Illinois, the Aberdeen Proving Ground, Maryland, and Lowry Field, Colorado. Deliveries to operational groups began in late 1937, the first being the 7th Bombardment Group at Hamilton Field, California.