a cobra? One with no teeth maybe. Seriously, the USA at this time has no real army, a navy that is mostly in the process of being built, and an air force not any better off. I suppose declaring war in '40 or '41 would kick start everything a couple years early... but the US still isn't going to be able to do much of anything for a while...
By the end of 1939 the USN had the following in commission:
5 fleet carriers, including the first two Yorktowns
15 battleships
18 heavy cruisers
19 light cruisers
over 150 destroyers
nearly 100 submarines
plus numerous auxiliaries, transports, etc.
That's "still being built" only by comparison with what the US finished the war with. It was a major fleet by 1940 standards, superior to Germany's in every class save possibly submarines.
During 1940 the US commissioned another carrier, 18 destroyers, and 5 submarines.
During 1941 the US commissioned yet another carrier, two battleships, a CVE, a light cruiser, 17 destroyers, and 13 submarines.
Edit to add: data from here:
http://www.ibiblio.org/hyperwar/USN/