The Martin PB2M/ JRM Mars was the largest flying boat to enter service with the US Navy, although only a handful were completed for use as a transport aircraft.
The prototype Martin Model 170 Mars was originally ordered on 23 August 1938 as the XPB2M-1 long range maritime patrol bomber. It was a high winged flying boat, powered by four engines. It was armed with nose and tail power operated turrets. It had a twin tail, with the tail turret between the two. Internally it had two decks and could carry a crew of 11 (including one duty and one reserve crew). The single prototype was launched on 5 November 1941, but didn’t make its maiden flight until 3 July 1942.
By this point the United States was already involved in the Second World War, and the Mars needed some significant changes to make it suitable for combat use (including adding some armour protection for the crew and extra guns). It was decided not to order the Mars as a patrol bomber. Instead the prototype was turned into a transport aircraft, as the XPB2M-1R. The gun turrets were removed and the gaps faired over. The fuselage was strengthened to allow it to carry heavy cargos.
The new transport entered service with the US Naval Air Transport Service in December 1943 and soon demonstrated its impressive capabilities. On its very first transport mission it flew 4,375 miles non-stop from Maryland to Natal, Brazil, carrying 13,000lb of cargo. On the return trip it carried a record payload of 35,000lb on a 1,216 mile stage. Early in 1944 it carried 20,500lb of cargo on a 4,700 mile round trip to Hawaii completed in 27 hours, 26 minutes.
In January 1945 the US Navy ordered twenty examples of a purpose built cargo version, as the JRM-1. This aircraft had a modified hull, a single fin and rudder and was powered by four 2,300hp R-3360-8 engines. It had a maximum take-off weight of 145,000lb, a top speed of 225 mph and carried two complete crews for four (a total of eight). Internally it had improved cargo handling facilities, including a winch with a 5,000lb capacity. It could also be converted into an air ambulance or into an assault transport, in which role it could carry 132 troops or seven jeeps.
Fifteen of the twenty aircraft were cancelled at the end of the war, and only five were completed as the JRM-1. On 19 May 1946 one of these aircraft set an unofficial record for the number of people carried in a single flight, taking 308 people from Alameda to San Diego.
One example of an improved version, the JRM-2, was also completed in the autumn of 1947, bringing the total number of aircraft up to seven. This had a maximum take-off weight of 165,000lb. The existing JRM-1s were modified to the new standard, and designated as the JRM-3. The aircraft were used by Squadron VR-2, at Alameda.
The Mars was retired from US Navy service in the mid 1950s. Some were then purchased by Forest Industry Flying Tankers Ltd and turned into forest fire fighting aircraft, complete with hull scoops that allowed them to lift 7,000 US gallons of water and drop it onto fires.
Engine: Four Wright R-3350-8
Power: 2,300hp each
Length: 120ft 3in
Gross Weight: 145,000lb
Maximum Speed: 225mph