The japanese 127/50 3rd Year Type that equipped the Fubuki class Destroyer is considered one of the first DP guns. The gun was designed in 1926 and entered service in 1928.
http://www.navweaps.com/Weapons/WNJAP_5-50_3ns.htm
Starting with the Ayanami subclass (Shikinami commissioned in 1929, another 4 ships following in 1930, 4 in 1931 and 1 in 1932) the gun model B had a maximum elevation of 75° making it a true DP mount in all aspects. (Yes, a bit outdated by the time WWII broke out, but in 1928/1930 this was one of the most modern guns around.)