The Martin-built Mars flying boats were the world’s largest operational seaplanes and the Navy’s largest World War II-era aircraft. Although designed to be patrol bombers, all served operationally as long-range transports.
Evolving from Martin’s experience with four-engine commercial “China Clipper” flying boats, the prototype XPB2M-1 was ordered in 1938; it was launched on 8 November 1941 but did not make its first flight until 3 July 1942. The plane was a graceful, high- wing, four-engine aircraft with fixed stabilizing floats under the wings, and a twin-tail design with large oval fins. It was designed to carry several tons of bombs or four aerial torpedoes and to be equipped with defensive gun turrets. (All production aircraft had a single tail fin with the gun positions faired over.)
The prototype “Mars” aircraft was not delivered to the Navy until November 1943, having been modified to a transport configuration and given the suffix “R”. The PB2M-1R was credited with a maximum speed of 221 m.p.h. and a range of 4,945 miles. It was flown by a crew of 11, and could carry 150 troops, plus relief flight crews. In November 1943, carrying 13,000 pounds of cargo, the aircraft flew nonstop from Patuxent River, Maryland, to Natal, Brazil, covering 4,375 miles in 28 1/2 hours.
With the Battle of the Atlantic raging in 1942, industrialist Henry J. Kaiser proposed building Mars flying boats to carry cargo over the Atlantic to escape U-boat torpedoes. Kaiser wanted to produce thousands of Mars flying boats as well as a follow-on, 200-ton version.
(The latter prototype flew as Howard Hughes’ Spruce Goose.) But the Navy could not pin down Kaiser on details and costs, and the program was stillborn.
The Navy ordered 20 Mars production JRM-1 aircraft from Martin. The designation reflected the utility/cargo role (J for utility, R for transport, M for Martin). In the event, only four JRM-ls were built, plus a single JRM-2, the last with more powerful engines. The aircraft were named for Pacific island groups: Hawaii Mars, Marianas Mars, Marshall Mars, and Philippine Mars. The single JRM-2 was named Caroline Mars.
They were to carry 132 fully-equipped troops, or 84 litters plus 10 medical attendants, or 11 tons of cargo. They had two troop/cargo decks with stairways forward and aft, separate crew quarters, and a 5,000-pound-capacity cargo crane under the port wing root.
With a gross takeoff weight of 145,000 pounds, the JRM-ls were the largest flying boats yet built; the single JRM-2 was a 165,000-pound aircraft that could carry 16 tons of cargo. The largest foreign flying boats were Britain’s post-war ten-engine (yes, ten) SR.45 Princess passenger planes that grossed 330,000 pounds. The first flew in 1952; three were built, but only the first had engines fitted, and it never entered service.
JRM deliveries to Transport Squadron (VR)-2 based at Naval Air Station Alameda, California, began in January 1945, and the Caroline Mars joined the fleet in April 1947.
The Hawaii Mars was lost while attempting to land in the Chesapeake Bay on 5 August 1945, three weeks after its first flight. The aircraft had lost the upper section of its vertical fin while in flight. Only one of the ten crewmen was injured, but the plane did not fly again.
The Caroline Mars set a number of records. On 28 August 1948, the aircraft flew from Honolulu to Chicago (landing on Lake Michigan), a non-stop flight of 4,748 nautical miles in 24 hours, 12 minutes; on 5 September 1948, it carried 68,282 pounds of cargo from Patuxent River to Cleveland, Ohio—the heaviest payload yet lifted by an aircraft; and on 19 June 1949 it flew from Honolulu to San Diego, carrying 144 passengers on the 2,609-nautical-mile flight.
The Marshall Mars also set records. On 19 May 1949, it lifted 301 passengers plus 7 crewmen on a flight from Alameda to San Diego—a world record for the number of passengers carried by air on a single flight. Sadly, it later was lost, catching fire while on a test flight and landing at sea three miles off Pearl Harbor on 5 April 1950. The seven-man crew suffered no casualties.
Despite their size, the Mars flying boats proved relatively easy to operate and maintain. In 1952, for example, the two remaining JRM-1 aircraft had an average availability of 92%, and the single JRM-2 was rated at 85%.
The Mars flying boats served the Navy until August 1956. The four operational planes carried more than 200,000 passengers, mostly between Hawaii and Alameda, thrice weekly. Their retirement was a harbinger of the departure of all flying boats from the U.S. Navy 11 years later. The surviving aircraft did have an after life: For more than 40 years, some flew as civilian water bombers fighting forest fires.
