In the 1930s and early 1940s, the Navy divided
carrier-borne bombers into two types: the
torpedo bomber and the
dive bomber, each with crews of two or three men. Wartime experience showed that pilots could aim bombs and torpedoes without assistance from other crewmembers as well as navigate with the aid of
radio beacons, and the development of more powerful engines meant that faster aircraft no longer needed a rear gunner for self-defense. Furthermore, the consolidation of the two types of bombers greatly increased the flexibility of a carrier's air group and allowed the number of fighters in an air group to be increased. In 1943, the US Navy invited proposals for a new multi-purpose bomber and selected four designs in September: the
Curtiss XBTC,
Douglas XBT2D Skyraider,
Kaiser-Fleetwings BTK and the Martin XBTM. Martin was tasked to provide a backup to the Curtiss design which had been selected as a replacement to the
Curtiss SB2C Helldiver. Due to the US Navy's concern that the Curtiss design was overly complex and that the company's record was particularly poor during the Helldiver's development, Martin was instructed to create an "unexperimental" design that would be a reliable platform for the
Pratt & Whitney R-4360 Wasp Major radial engine that powered both aircraft. Two prototypes were ordered from Martin on 31 May 1944 with the internal designation of
Model 210. The XBTM-1 was a low-wing, all-metal
monoplane with folding wings to allow more compact storage in carrier
hangar decks, and
conventional landing gear. Its fuselage was an oval-shaped stressed-skin
semi-monocoque with the single-seat
cockpit and its teardrop-shaped
canopy positioned just aft of the air-cooled engine. Just behind the cockpit was a fuel tank. The large wing consisted of a two-
spar center section with hydraulically folded three-spar outer panels. A large
dive brake was positioned on the trailing edge of the wing. When closed it could be lowered for use as a landing
flap or it could be split into alternating upper and lower sections, with intermeshing "fingers" for use in its intended role. It was very effective in this role, mainly due to its great surface area, but this was at the cost of the width of the
ailerons, which significantly reduced their efficiency. A pair of fuel tanks were positioned in the
roots of the center section. All fuel tanks were
self sealing and the pilot and
oil cooler were protected by of armor. The fixed armament of four
T-31 autocannon was fitted in the center section, adjacent to the outer wing panels with 200
rounds per gun. A centerline
hardpoint and a pair of outer hardpoints were installed on the center section and rated to take bombs, fuel tanks or torpedoes up to in weight. The outer hardpoints could also carry an AN/
APS-4 search radar in a pod. A dozen hardpoints could be installed on the outer wing panels to carry bombs or
High Velocity Aircraft Rockets. In service, the Mauler earned the nickname "Able Mabel" because its
AM designation and the fact that in the phonetic alphabet of the era the letter
A was pronounced as "Able", the name
Mabel being a rhyme and representing the M, and perhaps of its remarkable load-carrying ability, once lifting of ordnance (three torpedoes, a dozen 250-pound bombs plus its 20 mm guns and their ammunition) on 30 March 1949, perhaps the heaviest load ever carried by a single-engine, piston-powered aircraft. The first XBTM-1 made its
maiden flight on 26 August 1944 and began flight testing after it reached the
Naval Air Test Center on 11 December. The Navy ordered 750 more aircraft on 15 January 1945, although this was reduced to 99 aircraft after the
surrender of Japan in August. The second prototype made its first flight on 20 May. Initial flight tests conducted with the first two prototypes revealed significant problems with the engine, its
cowling, the
vertical stabilizer and
rudder. In response, the cowling was lengthened and the engine mount was canted two degrees to the right to offset the engine's tremendous
torque. The length of the
carburetor airscoop was extended and the propeller
spinner, rudder, and the vertical stabilizer were redesigned. In April 1946 the aircraft designation was changed to AM-1 when the Navy replaced its Bomber-Torpedo classification with Attack, well before the redesign was completed in early 1947. First deliveries began in March 1947 and a flight test program began that month that lasted three years before the major deficiencies identified were fully corrected. Carrier landing trials revealed a structural weakness of the rear fuselage when one aircraft was broken in half during a heavy landing. Severe vibrations in the tail upon engaging the
arresting wire were cured by adding a
roller bearing to the
tailhook to counter the sideways forces placed on the tailhook. Other necessary changes were the addition of
spoiler ailerons and an
elevator control boost to improve the aircraft's poor controllability at low speeds. In addition, the cockpit layout was unsatisfactory and had to be redesigned. The NATC finally deemed the Mauler acceptable for carrier landings in August 1948 even though aircraft had been issued to one
squadron earlier in the year and a new batch of 50 aircraft had been ordered in May. Despite all the modifications to the aircraft over its short life, it remained a maintenance nightmare, especially the leaky hydraulic systems. The AM-1Q was developed for
electronic countermeasures duties. The fuselage fuel tank was removed to make room for the electronics operator and his equipment in a windowless compartment. The aircraft carried several radio and radar
receivers, transmitters and a
signal analyzer. The operator could also drop bundles of
chaff through a chute to block radar signals. Pilots found the Mauler a heavy-handling aircraft which was difficult to fly in formation and hard to land aboard a carrier because a less-than-perfect landing often caused the aircraft to bounce over the arresting wires and into the safety barrier. It was a very stable dive bomber, more so than the Skyraider, and could carry more ordnance. Maintenance problems and the difficulty of landing aboard a carrier caused some pilots to give it the nickname of "Awful Monster". ==Operational history==