This genre is known as mecha musume. While some anime TV shows (most notably Strike Witches) use this genre, it usually is more of a "fan art" type of thing. You'll more often find amateur Japanese artists who are very proficient in drawing anime/manga style art do this sort of thing than actual professional artists working at an anime studio, at least based on what I've seen online so far. One site that seems to specialize in this genre is http://girlarms.exblog.jp/i8/
From my observation, most seem to be based on aircraft and tanks. I've uploaded all the pictures found on that gallery page of that website to the image host site Tinypic and posted them here for all to see.
I'd appreciate it if people left comments on the genre as a whole and/or on individual pictures I've posted here.
Panzer IV (German tank)
Karl 60cm Morser (German artillery)
JS-3 Stalin (Soviet tank)
CM-32 LAV P2 (Chinese armored troop carrier)
Sukhoi Su-47 (Russian experimental fighter jet)
FW Triebflugel (German experimental VTOL design, never built)
Schwere Gustav (German train car mounted artillery)
Renault Char B1 (French tank)
SR-71 Blackbird (American supersonic spy plane)
T-35 (Soviet tank)