TECHNICAL SPECIFICATIONS (PBM-1) Type: Seven- to ten- place patrol- bomber flying boat
(amphibian).
Manufacturer: Glenn L. Martin Co., Baltimore, Mary- land.
Total produced: 1,366 (USN, USCG)
Powerplants: Two 1,600-hp Wright R-2600-6 Twin Cyclone 14-cylinder air- cooled radial engines driving three- bladed, electric controllable- pitch metal propellers.
Armament: One .50-cal. machine gun in powered nose, dorsal, and tail turrets, one .50-cal. machine gun in each waist position, one .30-cal. machine gun firing downward in aft tunnel position, and up to 4,000 lbs. of bombs, torpedoes, or depth charges carried in nacelle bays.
Performance: Max. speed 200 mph; ceiling 20,200 ft.; range 3,424 mi. max.
Weights: 24,143 lbs. empty, 56,000 lbs. loaded.
Dimensions: Span 118 ft., length 77 ft. 2 in., wing area 1,400 sq. ft. The Martin PBM, in terms of general design and structure, was arguably the most advanced twin- engine flying boat of its day. But the real impetus behind the design had been economic: faced with financial losses on its M130 airliner and dismal prospects for commercial sales, Martin had little choice but re- turn to the highly competitive arena of military patrol boat con- tracts. Since Consolidated commanded such a strong position A Navy PBO-1 seen in AAF paint scheme in early 1942. This was
the first land-based Navy bomber since the T2D-1 of 1927. A PBO-1 of VP-82 scored first American sinking of an enemy sub- marine on March 1, 1942.
in the twin- engine market with its PBY, Martin sought to interest the Navy in a pro- posal for the four- engine model 160. Coin- cidentally, BuAer had already contracted for two other four- engine flying boat prototypes (see Consolidated XPB2Y-1 and Sikorsky XPBS-1), and Martin hoped to insert itself into the eventual competition for a produc- tion contract. As an unsolicited private ven- ture, however, Martin would be obliged to fund development of a prototype, which it was in no position to do; instead the com- pany planned to build a one- fourth (25 per- cent) scale flying demonstrator that would validate the main characteristics of the de- sign. When BuAer declined to consider an- other four- engine project, Martin went back to the drawing boards and returned in early 1937 with an all- new proposal for the twin- engine model 162, which the company promised would deliver substantially better speed, range, and payload than the twin- engine PBY. Consolidated’s president, Reuben Fleet, disputed Martin’s perform- ance claims and threatened the Navy with political repercussions if his company lost a production contract. As a compromise, BuAer awarded Martin a development con- tract the following June for a single XPBM- 1prototype with the understanding that fu- ture production plans would hinge on test results.
Timing was critical: construction of the XPBM-1 prototype would take eighteen months or longer, and Martin feared this in- terval might give Consolidated the oppor-
Above: A Martin XPBM-1 in original configuration, as seen in
1939 with straight horizontal stabilizer. Testing revealed a tail flutter problem remedied by adding a dihedral to the horizontal stabilizers, thus producing the “pinwheel” tail. Right: The Mar- tin M162A “Tadpole Clipper” parked under the port wing of the XPBM-1. This three-eighths (37.5 percent) scale flying testbed was intended to prove the design concept upon which the full- size aircraft would be based.
tunity to introduce an altogether new twin- engine prototype (i.e., Consolidated Model 31/P4Y). To speed progress, Martin’s engineering staff reworked the plan for the M160 demonstrator into a three- eighths scale (37.5 percent) replica of the XPBM- 1, and as the M162A, had it flying before the end of the year. Despite receiving a production contract for twenty- one PBM- 1s in late December 1937, which had achieved the main purpose of a demonstrator, Martin continued the M162A test program and in doing so, obtained much valuable data subsequently in- corporated into the final arrangement of the XPBM-1 hull.
When rolled- out for its first flight on February 18, 1939, the XPBM-1 represented a state- of- the- art flying boat concept. The design attained new levels of aerodynamic efficiency by mounting the fully cantilevered wings to the upper hull in a gull configuration which raised the engines well above the sea- spray without need for drag- inducing struts or pylons. Aft of the rear step, the hull curved upward to support a high- mounted twin- fin empennage designed to provide adequate directional stability and single- engine control when needed. To keep the airframe clean, droppable munitions were housed in enclosed nacelle bays and the stabilizing floats folded into the wings so that only one side remained exposed to the slipstream. A tail flutter problem revealed in early testing was cured by adding an amount of dihedral to the horizontal stabilizers that matched the gull angle of the inner wings, thereby giving the type its distinctive “pinwheel” tail. Performance trials indicated a 17 mph increase in top speed over the PBY-4, but the real difference was that the PBM-1 would go 30 percent further car- rying twice the load of bombs or depth charges. And defensive armament was the most formidable yet seen on any twin- engine patrol boat: powered nose and dorsal turrets each armed with a .50-caliber machine gun, flexible .50-caliber guns in two waist positions and in the tail, plus a .30-caliber tunnel gun firing downward in the aft fuselage.
PBM-1s initially became operational during the fall of 1940
with VP-55 and VP-56, based at Norfolk, Virginia, then after combining as VP-74 in mid–1941, moved to a new base in Bermuda. But even as deliveries of the first PBM-1s started, the Navy was making plans for unheard of levels of aircraft production and between November 1940 and August 1941, as part of the buildup, Martin received a series of contracts for 379 aircraft to be delivered as the PBM-3. The sole XPBM-2, a long- range version with increased fuel tankage and stressed for catapult launches, was tested during 1941 but never placed in production. As of December 1941, the Navy had the XPBM- 1and XPBM-2, plus 20 operational PBM-1s on hand. Deliveries of PBM-3s commenced in the spring of 1942, and ultimately, 1,344 more of the type were manufactured, consisting of 581 PBM-3s in four sub- variants, 729 PBM-5s (upgraded to Pratt & Whitney R-2800 engines), and 34 PBM-5A amphibians, the final examples being delivered in 1949. Twenty- seven PBM-3s were assigned to the Coast Guard in 1943, followed by 41 PBM- 5s in 1944.