you'll get few hits from 4E & 2E bombers, using Naval Attack. they'll bring better hurt against docked ships, when set to Port Attack (where they use their GrdB skill). so you are better off conserving your Phillipines bombers, then unleashing them against a port (such as Legaspi) when the anchor symbol shows that enemy ships are docked there.
1E DBs w/ 1000# bombs are the real ship-killers, they are the ones you must train in NavB (train NavB at 5k', attack at 10-15k' to get the DB advantage).
later in the war, the Allies will get 'Attack Bombers', such as the B-25D1, B-25G, & A-20G. these guys are nasty, they carry a full bomb-load at low-level (non-attack bombers carry a reduced bomb-load at low level). so you want to train their pilots at 1k' (NavB or GrdB depending on your desired target), then train them more at 100' (either NavB or GrdB) for strafing skill. when set to attack at 1k', they'll bomb the target, then strafe to suppress flak. their 500# bombs can mung IJ CAs w/ deck-penetration hits.
& don't forget that USN Cats can carry 2 torps each. grab 1 or 2 of your PBY groups and train them for NavT. they might help you defend Port Moresby! but be careful how you task them - if the group mission is Naval Search, they'll only carry bombs. you must set the mission to Naval Attack (using torps), then you can set a %age of the group to search, leaving the remainder un-assigned for possible strikes. also, you'll need a friendly airHQ to provide the torps.
but all bets are off if you try to strike into heavy enemy fighter defenses without your own fighter escort. if you expend your highly-trained strike pilots and airframes, your efforts will be wasted.