Surprisingly, given the sudden stops that were likely to occur in a ditching or carrier-landing arrestment (not to mention being stopped by the barrier), U.S. Navy carrier airplane seats did not have shoulder harnesses before mid 1942.
It may be that ditchings and crashes in the years between the world wars may have been relatively rare, although a black eye, missing teeth, or a scar on a naval aviator's forehead from encountering his gun sight or instrument panel was not unusual. Shoulder harness were also thought by the Bureau of Aeronautics (BuAer) to impede escape in the event of a crash or perhaps considered by the pilots to be a nuisance in using the early gun sight or the plotting board that slid out of the instrument panel, which required leaning forward. (The U.S. Army also did not adopt shoulder harnesses before the war but the Brits did.)
Jim Maas, who is the subject matter expert for the Brewster F2A Buffalo, reports that a BuAer change order to install shoulder harnesses in it was issued on 12 June 1942, to be accomplished "as soon as practicable" but no later than the next major overhaul. According to the January 1948 issue of Flying Magazine, the requirement to modify F4Fs to add shoulder harnesses was issued on 18 June.
The following article that appeared in the 15 June 1943 issue of Naval Aviation News suggests implementation took time:
An article in the 1 January 1944 issue of Naval Aviation News suggests that shoulder harness implementation was still being driven at the local level (VC-19 was a composite squadron operating FM Wildcats and TBM Avengers off escort carriers in the north Atlantic by late 1943).
More information on shoulder harness implementation would be welcomed.