American twin-engined heavy bomber aircraft in service 1936-1946
The Douglas B-18 Bolo is an American twin-engined heavy 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, on 22 August 1942 in the Caribbean. 
Design and development.
In 1934, the United States Army Air Corps requested for a twin-engine heavy 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 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.
Operational history.
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.
Production B-18s, with full military equipment, had a maximum speed of , cruising speed of , and combat range of . By 1940, most USAAC bomber squadrons were equipped with B-18s or B-18As.
However, the B-18/B-18A's deficiencies were made apparent when an all-red Soviet Ilyushin TsKB-30 named "Moskva" (a prototype for the twin-engine DB-3 which flew the same year as the B-18) made a non-stop flight from Moscow to North America in April 1939, a distance of , which was well beyond the capabilities of the B-18. The TsKB-30/DB-3 was also 25% faster, was capable of carrying a bomb load 2.5 times as large as the B-18, and carried a heavier defensive armament. In August of the same year, a Japanese Mitsubishi G3M2 named "Nippon" (which also had its first flight the same year as the B-18) flew from Tokyo to the US, and then around the world, with the stage from Chitose, Hokkaido to Nome, Alaska being over . The military version (code named "Nell" during WW2) could also carry more than the B-18, further, faster, and was also better armed. Both types had roughly higher service ceilings as well.
The Air Corps conceded that the Bolo was obsolete and unsuitable for its intended role. However, in spite of this, the B-18/B-18A was still the most numerous American bomber type deployed outside the continental United States at the time of the attack on Pearl Harbor. The B-18 would be a stopgap until the more capable Boeing B-17 Flying Fortress and Consolidated B-24 Liberator became available in quantity.
World War II.
When war came to the Pacific, most of the B-18/B-18A aircraft based overseas in the Philippines and in Hawaii were destroyed on the ground in the initial Japanese onslaught. The few Bolos that remained played no significant role in subsequent operations.
The B-18s remaining in the continental US and in the Caribbean were then deployed in a defensive role in anticipation of attacks on the US mainland. These attacks never materialized. B-17s supplanted B-18s in first-line service in 1942. Following this, 122 B-18As were modified for anti-submarine warfare. The bombardier was replaced by a search radar with a large radome. Magnetic anomaly detection (MAD) equipment was sometimes housed in a tail boom. These aircraft, designated B-18B, were used in the Caribbean on anti-submarine patrol. On 2 October 1942, a B-18A, piloted by Captain Howard Burhanna Jr. of the 99th Bomb Squadron, depth charged and sank the north of Cayenne, French Guiana.
Two aircraft were transferred to the Brazilian Air Force in 1942, and were used with a provisional conversion training unit set up under the provisions of Lend-Lease. They were later used for anti-submarine patrols. They were struck off at the end of the war.
In 1940 the Royal Canadian Air Force acquired 20 B-18As (as the Douglas Digby Mark I), and also used them for patrol duties, being immediately issued to 10 Squadron to replace the squadron's Westland Wapitis.
Bolos and Digbys sank an additional two submarines during the course of the war. RCAF Eastern Air Command (EAC) Digbys carried out 11 attacks on U-boats. was confirmed sunk by Flying Officer F. Raymes' crew of No. 10 (BR) Squadron, on 30 October 1942. east of Newfoundland. However, the antisubmarine role was relatively short-lived, and the Bolos were superseded in this role in 1943 by B-24 Liberators, which had a much heavier payload and a substantially longer range, which finally closed the mid-Atlantic gap. Some of the Douglas Digbys in Canadian service were converted into transports or used for training.
Surviving USAAF B-18s ended their useful lives in training and transport roles, and saw no further combat action. Two B-18As were modified as unarmed cargo transports under the designation C-58. At the end of the war, remaining examples were sold as surplus on the commercial market. Some postwar B-18s were operated as cargo or crop-spraying aircraft by commercial operators.
Manufacturer's designation for prototype, first of B-18 production run, 1 built.
Initial production version, 131 or 133 built.
Trainer B-18 with bomb gear removed.
 Manufacturer's designation for prototype with powered nose turret; last of B-18 production run, 1 built.
B-18 with more powerful Wright R-1820-53 engines and relocated bombardier's station, 217 built. Manufacturer's designation was DB-4.
Trainer B-18A with bomb gear removed.
 Antisubmarine conversion, 122 converted by adding a radar and magnetic anomaly detector.
Antisubmarine conversion, 2 converted. Fixed forward-firing machine gun, starboard side of the fuselage near lower nose glazing.
Improved B-18 with Wright R-2600-3 radial engines. Not built, due to better designs being available.
Transport conversion.
Royal Canadian Air Force modification of B-18A. Named for RAF Digby.
 1st Bomber Group (3 examples)
 No. 10 Squadron RCAF, Halifax, Nova Scotia (Digby Mk.1)
 1st Search Attack Group, Langley Field, Virginia (B-18A/B/C)
 2d Bombardment Group, Langley Field, Virginia (B-18A)
 3d Bombardment Group, Barksdale Field, Louisiana (B-18)
 5th Bombardment Group, Hickam Field, Hawaii (B-18)
 6th Bombardment Group, Rio Hato Airfield, Panama, (B-18/B-18A/B)
 7th Bombardment Group, Hamilton Field, California, (B-18)
 5th Bombardment Group, Luke Field, Oahu, Hawaii Territory (B-18)
 9th Bombardment Group, Caribbean; Panama and South American air bases (B-18/B-18A/B)
 11th Bombardment Group, Hickam Field, Hawaii Territory (B-18)
 13th Bombardment Group, Langley Field, Virginia (B-18A/B)
 17th Bombardment Group, McChord Field, Washington (B-18)
 19th Bombardment Group, Clark Field, Philippines Commonwealth (B-18)
 22d Bombardment Group, Muroc Field, California (B-18)
 25th Bombardment Group, Caribbean (B-18/B)
 27th Bombardment Group, Barksdale Field, Louisiana (B-18)
 28th Bombardment Group, California, (B-18)
 28th Composite Group, Elmendorf Field, Alaska, (B-18A)
 29th Bombardment Group, Langley Field (B-18A)
 40th Bombardment Group, Panama, Puerto Rico (B-18/B)
 41st Bombardment Group, California, (B-18)
 42nd Bombardment Group, Portland, Oregon (B-18)
 45th Bombardment Group, Savannah Airfield, Georgia (B-18A)
 47th Bombardment Group, McChord Field, Washington (B-18)
 479th Antisubmarine Group, Langley Field, Virginia (B-18A/B)
Aircraft on display.
Six B-18s are known to exist, five of them preserved or under restoration in museums in the United States, and one is a wreck still located at its crash site:
Specifications (B-18A).
"Data from" McDonnell Douglas aircraft since 1920 : Volume IGeneral characteristics* Crew: Six* Length: * Wingspan: * Height: * Wing area: * Airfoil: root: NACA 2215; tip: NACA 2209* Empty weight: * Gross weight: * Max takeoff weight: * Fuel capacity: * Powerplant: 2 × Wright R-1820-53 Cyclone 9-cylinder air-cooled radial piston engines, * Propellers: 3-bladed fully-feathering Hamilton Standard Hydromatic propellers
Performance* Maximum speed: (, ) at* Cruise speed: (, ) * Range: (, ) * Ferry range: (, )* Service ceiling: * Time to altitude: in 9 minutes 54 seconds* Wing loading: * Power/mass: 
Armament
Avionics
See also.
Related development
Aircraft of comparable role, configuration, and era
Related lists
References.
Notes.
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Citations.
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Bibliography.
[[Category:1930s United States bomber aircraft]]
[[Category:Douglas aircraft|B-18 Bolo]]
[[Category:1930s United States military trainer aircraft]]
[[Category:1930s United States military transport aircraft]]
[[Category:Aircraft first flown in 1935]]
[[Category:Mid-wing aircraft]]
[[Category:Twin piston-engined tractor aircraft]]