Navy Patrol Flying Boats, 1920-1932
PN-10, PN-12, XPB-2, PD-1
The first flying boat built since 1919 by a private company to a Navy design was the Boeing PB-l ordered September 23, 1924, for the same trans-Pacific flight as the PN-9. Two big 800-hp Packard 2A-2540s back-to-back between the hull and upper wing, turned one tractor and one pusher propeller. First flown at Seattle July 29, 1925, the PB-l had five cockpits, the Clark Y airfoil, and a new composite hull design with metal bottom and plywood deck. Three Lewis guns and up to 4,000 pounds of bombs could be carried, but engine difficulties prevented participation in the Hawaii flight.
The next pair of boats were the PN-l0s with geared 525-hp Packard 2A-1500s, and three-bladed propellers. They were expected to introduce a metal wing structure covered by fabric, but delay of the duralumin parts caused the first flight on June 21, 1926, to be made with wooden wings. Distance records set in August 1927 by a PN-10 at San Diego included one of 1,569 miles in 20 hours, 15 minutes, with an 1,100-pound load. Two more PN-l0s were ordered with the metal wings and the first flew November 30, 1927, the first Navy flying boat with an all-metal structure, although cotton fabric still covered wings and control surfaces.
Up to this point, water-cooled engines had been used on Navy patrol planes, but in 1928 the Navy changed to air-cooled radials on flying boats. The second PN-10 became the XPN-12 when fitted with 525-hp Wright R-1750 Cyclones in December 1927, and the same engines powered the boat ordered as the fourth PN-10, but delivered in June 1928 as a PN-12.
Pratt & Whitney R-1690 Hornets were mounted on the other two PN-10s, which were also redesignated PN-12, and set an altitude record in June 1928 of 15,426 feet with 4,400 pounds. Boeing’s ship was also rebuilt at the NAF as the XPB-2 with two Pratt & Whitney R-1690 Hornets, instead of the unsatisfactory Packards, and was test flown in April 1928.
The first PN-12 version with Wright engines was the prototype for the first production flying boats ordered since the war. Twenty-five Douglas PD-ls ordered December 29, 1927, entered service in June 1929 with VP-7 at San Diego, and were then all concentrated at Pearl Harbor with VP-4 and VP-6. Armament comprised two flexible guns and four 500-pound bombs, with automatic pilot and radio equipment.
On May 31, 1929, the Navy ordered 25 Martin PM-l and three NAF XP2N boats. Martin’s contract was increased to 30 PM-ls delivered at Baltimore from July to October 1930 with 525-hp Wright R-1750D Cyclones, but the last three boats were diverted to Brazil, and replaced by PM-2s. Six each went to Scouting Force squadrons VP-2 at Coco Solo and VP-8, VP-9, and VP-10 based on tenders Argonne and Wright.
The PM-ls were almost identical to the Douglas PD-ls except for rounded engine nacelles, and had four .30-caliber guns in the forward cockpit, rear cockpit, and side openings. Four 500 or two 1,000-pound bombs could be carried under the wings, and a 239-pound radio and a five-man life raft were standard equipment.