Trying to back out numbers here ...
I don't find a total size of the US Army Air Corps' European flight crew operations, but I do find a casualty count of 70,000 (<https://www.nationalww2museum.org/war/articles/black-thursda...>). If that constitutes 75% of the total force size, then we come up with a total force size of 93,333. Let's call that 100,000.
For the merchant marine:
During World War II, nearly 250,000 civilian merchant mariners served as part of the U.S. military and delivered supplies and armed forces personnel by ship to foreign countries engulfed in the war. Between 1939 and 1945, 9,521 merchant mariners lost their lives — a higher proportion than those killed than in any military branch, according to the National World War II Museum.
<https://www.defense.gov/News/News-Stories/Article/Article/30...>
The US Army Air Corps trained 435,000 pilots during WWII, with more trained by the US Navy and Marines.
"WWII’s Tragic Aviation Accidents", Warfare History Network (2018)
<https://archive.is/718ZJ#selection-671.287-671.295>
The US Navy reports 5,563,507 officers and 6,570 enlisted (totals are regular and reserve), presumably aviation, with 12,133 total fatalities, of which just under 30% were combat deaths. (The remainder were largely non-combat crashes.)
<https://www.history.navy.mil/research/library/online-reading...>
(The total officers count seems suspiciously high, and I suspect is for the total Navy corps, rather than just flight operations.)
And about 15,000 pilots died during training.
<https://www.realclearhistory.com/articles/2019/02/12/stagger...>
This still doesn't break out bomber crews relative to the overall US air operations, but at least for ballpark estimates, it seems that air crews and merchant sailor contingents were at the very least comparable in size, if not more air crews. That makes some sense as ships would be larger, fewer in number, and with a more efficient use of personnel (in terms of crew to net operations and tonnage delivered).
Another triangulation: the US produced about 300k combat aircraft, and about 35,000 bombers, during WWII:
<https://en.wikipedia.org/wiki/United_States_aircraft_product...>
(The story of industrial production and scale at which the US poured out materiel, whether ships, tanks, fighters, bombers, or what have you, during WWII, is worth its own discussion.)