Weight

Weight defines the, well, weight of your bomber. The engines will support only a certain amount of weight, so you will have to upgrade engines to higher levels in order to get more armor and other things.

Engines
There are three types of engines: It is not recommended to make your landing gear engines (Inner) lightweight until later tiers of the game, or landing gear will take extensive damage during a mission, with each one dying in about 6 shooting runs. (Each engine tier ALWAYS adds more weight limit and more armor.)
 * Standard engines-Normal engine type, add 500 or so weight limit to your craft each.
 * Armored engines-Give less weight limit than standard engines, but are more armored and so take more hits before breaking or being rendered inoperative.
 * Lightweight engines-Very powerful engine type, but have barely any armor whatsoever. Gives about 1250+ from the last lightweight engine type weight limit.

Make lightweight engines your outer engines as soon as possible, to get the most possible weight limit and allowing you to focus your money on crew gear and bomber upgrades before having to upgrade again.