The Douglas B-18 Bolo was a military adaptation of the DC-2 commercial transport to the long-range bombing role. Although totally obsolescent by the end of 1941, it was numerically the most important long-range bomber in service with the USAAC at the time of America's entry into World War 2.
The origin of the B-18 can be traced back to the May 1934 competition for a multi-engined bomber capable of carrying a ton of bombs at more than 200 mph over a distance of 2000 miles. This Army requirement envisaged from the start that the winning design would have a production run of as many as 220 planes. Several manufacturers were invited to submit bids, with the entries to be flown at Wright Field in a final competition to select the winner.
The Douglas entry drew heavily on the company's experience with its DC-2 commercial airliner. It was designed around the wings of the DC-2 and was fitted with a deeper and fatter fuselage which contained a bomb bay within its center section. The new bomber had larger tail surfaces than did the standard DC-2, plus a wing with a slightly larger span and area resulting from the fitting of rounded tips. A six-man crew was carried (two pilots, one navigator/bombardier, plus 3 gunners). Defensive armament consisted of three 0.30-cal machine guns, one each in manually-operated nose and dorsal turrets, and one firing from a ventral hatch. The dorsal turret was located just ahead of the vertical fin and was fully retractable. It was rather unusual in having a rectangular top, so that it could lie flush with the upper fuselage when retracted. A 4400-pound bombload could be carried in the bomb bay.
The aircraft made its first flight in April of 1935, powered by a pair of 850 hp Wright R-1820-G5 air-cooled radials. It was delivered to Wright Field for the competition in August of 1935. Competitors included the Martin 146, which was a streamlined and enlarged version of the B-10 twin-engined light bomber then already in Army service, plus the four-engined Boeing 299, which was eventually to emerge as the B-17 Flying Fortress.
Test flights proved the Douglas to be inferior in almost every respect to the Boeing 299. However, it did have the advantage over the Boeing design in being substantially cheaper, and on January 28, 1936 the Army ordered 82 B-18s, with the order being increased to 132 by June. Production B-18s were powered by a pair of 930 hp Wright R-1820-45 radials housed in revised cowlings. The nose cone was somewhat shorter than that of the prototype, and it contained more lateral windows as well as a bomb-aiming window in its forward lower portion.
The first production B-18 was delivered to Wright Field on February 23, 1937. The prototype was brought up to full B-18 standards and was redelivered to the Army five days later as serial number 37-51.
Douglas B-18 Bolo
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 in Maryland, and Lowry Field in Colorado. The first operation unit to receive the B-18 was the 7th Bombardment Group based at Hamilton Field in California. B-18s later went to the 5th Bombardment Group at Luke Field, Oahu, the 19th Bombardment Group and 38th Reconnaissance Squadron at Mitchell Field, and the 21st Reconnaissance Squadron at Langley Field, Virginia.
Specification of Douglas B-18
Two Wright R-1820-45 air cooled radials, rated at 930 hp for takeoff and 810 hp at 10,200 feet. Maximum speed 217 mph at 10,000 feet. Cruising speed 167 mph. Landing speed 64 mph. Service ceiling 24,200 feet. Absolute ceiling 25,850 feet. Initial climb rate 1355 feet per minute. An altitude of 10,000 feet could be attained in 9.1 minutes. Range was 1082 miles with 2200 pounds of bombs and 412 gallons of fuel, or 1200 miles with 4400 pounds of bombs and 802 gallons of fuel. Maximum ferry range was 2225 miles. Dimensions: wingspan 89 feet 6 inches, length 56 feet 8 inches, height 15 feet 2 inches, wing area 959 square feet. Weights: 15,719 pounds empty, 21,130 pounds gross, 27,087 pounds pounds maximum takeoff. Normal bombload was 2200 pounds, but a maximum bombload of 4400 pounds could be carried. Armed with three 0.30-inch machine guns in nose, dorsal, and ventral positions.
Douglas B-18A Bolo
The B-18A differed from the B-18 in having the bomb-aimer's position moved upward and forward underneath an extended glazed housing, while the flexible forward-firing nose gun was moved further back and below and was mounted inside a globular ball turret. This led to the rather unusual geometry in which the bombardier sat above and ahead of the nose gunner. A transparent domed cap was added to round off the top of the dorsal turret, so that it no longer lay flush with the fuselage when retracted. The B-18A was powered by two 1000-hp Wright R-1820-53 radials driving fully-feathering propellers. 177 B-18As were ordered on June 10, 1937, with 78 more being added to the contract on June 30, 1938. The B-18A flew for the first time on April 15, 1938. The first B-18A was delivered to the Army in April of 1938, with the last example being delivered in January of 1940. Only 217 out of the 255 ordered were actually delivered as B-18As, the last 38 examples being built as B-23s.
Specification of Douglas B-18A
Two Wright R-1820-53 air cooled radials, rated at 1000 hp for takeoff and 850 hp at 9600 feet. Maximum speed 215.5 mph at 10,000 feet. Cruising speed 167 mph. Landing speed 69 mph. Service ceiling 23,900 feet. Absolute ceiling 25,600 feet. Initial climb rate 1030 feet per minute. An altitude of 10,000 feet could be attained in 9.9 minutes. Range was 1150 miles with 2496 pounds of bombs. Dimensions: wingspan 89 feet 6 inches, length 57 feet 10 inches, height 15 feet 2 inches, wing area 959 square feet. Weights: 16,321 pounds empty, 22,123 pounds gross, 27,673 pounds pounds maximum takeoff. Normal bombload was 2200 pounds, but a maximum bombload of 4400 pounds could be carried. Armed with three 0.30-inch machine guns in nose, dorsal, and ventral positions.
Douglas B-18B Bolo
In 1942, 122 B-18As were modified for the maritime reconnaissance bombing role to counter the U-boat menace. These modified aircraft were redesignated B-18B. An SCR-517-T-4 ASV (air to surface vessel) radar set was mounted under a radome in the nose, replacing the bombardier's shark-nose glazed area. The bombardier's station was moved below and behind the radome, where the forward turret had formerly been located. In addition, a Mk IV Magnetic Anomaly Detector (MAD) set was installed in a long tubular boom that extended behind and below the the rudder. Some B-18Bs were also equipped with a set of retro bombtracks underneath the wings which could fire bombs backwards in a prearranged pattern.
B-18Bs are credited with two U-boat kills—U-654 on December 22, 1942 and U-512 on January 2, 1943. The antisubmarine role was relatively short lived. Surviving USAAF B-18s ended their useful lives in training and transport roles within the continental USA, and saw no further combat action. At the end of the war, those bombers that were left were sold as surplus on the commercial market. Some postwar B-18s of various models were operated as cargo or crop-spraying aircraft by commercial operators.