。
Boeing developed a flying boat for the Maritime Self-Defense Force.
Literal
Boeing-company [topic-は] maritime-self-defense-force-use [gen-の] flying-boat [obj-を] developed [polite-ました].
The suffix 用 ('for the use of X') attaches to nouns to form adjectival modifiers: 自衛隊用 'for the Self-Defense Force,' a compact way to spell out purpose or intended user. 海上自衛隊 decomposes as 海上 'maritime' + 自衛隊 'Self-Defense Force.' 飛行艇 is the specific term for a fixed-wing aircraft designed to take off and land on water (distinct from 水上機 'seaplane' with detachable floats). 社 is the standard suffix for 'company.'