300lb M31 GP in fact it appears to be 276lb real weight seems to have been a common bomb of the 30's, all planes seem to have been designed for it. From my research US seems to only have a comparable weight bomb in 1943 : 250lb GP bomb M57 model.
600lb M32 seems to have been also a common bomb in 30's and early war, it was employed by B-17s, there are several mission reports on web of B-17 dropping 600lb bombs in Pacific and Europe.
These were old bombs and not build anymore from what i could research.
A primer on US bombs ,it is from 1945:
http://ibiblio.org/pha/USN/Bombs.pdf
For example:
"On December 10, 1941 took off from San Marcelino Airfield piloted by Major Emmett "Rosie" O'Donnell before dawn and flew to Clark Field. On the ground, the B-17 was armed with eight 600 lbs bombs and departed alone to bomb Japanese shipping off Vigan."
http://www.pacificwrecks.com/aircraft/b-17/40-3061.html
Midway in 1942:
Preliminaries to the Battle of Midway opened on the afternoon of 3 June, when at 1623, nine B-17E's led by Lt. Col. Walter C. Sweeney, Jr., of the 431st Bombardment Squadron surprised the transport force with its supporting craft some 570 miles west of Midway, dropping 36 x 600-lb. demolition bombs from 8,000 feet.
It is possible that in places like Philippines, Hawaii had many of these more older bombs.