IJN and USN are best for going all out bomber. 

hmm...from my experience I'd have to say soviet. IJN was okay, but with no fighters, the IJN cvs don't have powerful AA until pcv. US is good, but you really want some fighters. 

Soviet bombers are pretty tough, and the ships have good gun slots for defense/AA.  

Why would you even think of making a USN BW? USN TBW works, but if you're new to CV like that it's totally defeating the purpose. Same with KM, incidentally.

IJN is probably the best BW nation overall, or RN TB if you love TB for some reason. SN DBW works too.

You NEED fighters.   Repeat...you NEED fighters.

Fighter escort for your bombers helps them get thru.   If your fighters get to the enemy line before the bombers, the AA ships will concentrate on them first and that increases the chances of your later arriving bombers of getting thru.

In later stages of the game, when the enemy has mostly surviving CVs, you'll have a tough time getting your bombers thru their fighter screen.   Their fighters will be faster then your bombers.
Then they will send bombers after you...and how will you protect yourself?

Any CV player that uses a lot of bombers finds out that they NEED fighters.

All round best bomber nation by far is IJN. Weak fighters are offset by speed which is far more useful for a BW player then powerful fighters. Overall IJN has good powerful DBs, useable TB, good plane mobility, good bomber pilots, fast fighters suitible for escort and distraction duties, not to mention all kinds of fun joke CVs like the Ise BB Hybrid and the Mog/Oyo CVs. I really do regret waiting all this time to finally start IJN.

If you want to bw... use SN, ijn is a good choice aswell but sn is the best

BWing requires you to apply your personal taste and playstyles,
Therefore:

US: Fast bombers, lower damage, decent defense vs AA/FPs

UK: Fastest Bombers, lowest damage, decent defense against AA, best defence vs FPs

IJN(Japan): Slightly slower bombers, good bomb damage, bad defense against AA, ok vs. FPs

KM(Germany): Decent speed, decent bomb damage, bad general defense vs. FPs,
(NOTE: KM Bombers are given a DESERT camo, making them REALLY easy to spot vs. other bombers)

MN(France)(My Personal Favorite): Normal speed, 3rd best damage, decent against AA,  can shoot down up to T3 FPs with little problems (If correctly classed), hard to spot.

SN(Russia): Slowest Bombers, most powerful bombs, heavy armour VS. AA, crap vs. FPs.

RM(Italy): Slow, low damage, higher fuel cap(stays in the air longer), weak against AA and FPs.
(NOTE: RM Bombers ALSO use a DESERT camo, making them very easy to spot.) 



My Reccomendations:
Avoid Italy for a BW at all costs, they are just pathetic in every way.

US bombers look like FPs, and will scare away other peoples FP sometimes XD

UK bombers are fast enough to outrun some fighters, and avoid AA

French bombers are a great blend of all of the stats, and have the ability to shoot down other fps at high levels.

SN are the tanks of bombers, but are VERY slow, they wont outrun anything.

Japanese bombers are smaller than other nations bombers, making them hard to hit with AA.

LEARN TO MANUAL DB ASAP, PM me if you want some lessons. :P 

Im considering being a TorpedoBW, It easier to avoid AA fires because i dont need to go to max altitude .

I looked at trainworld.us but theres no data on bomb data of SN,MN and Italy ..


hmmm maybe Im going RN if I want to use DB, but you say it has the lowest damage, so it has the 'lowest' but is that far from the damage of the 'average' damage