Have you talked to your vet about your concerns? Boxers are very slim and trim dogs. My girl isn't quite 2 years old, she's 22" tall, weighs about 46lbs, and looks good. You can just barely see her ribs.
What concerns me a little is that you said you can see her hip bones. Are they very prominent, or just barely visible?
Has she been checked for worms or de-wormed at all? If she has parasites, they can make your dog thinner, and skinny because she isn't getting the nutrition she should be.
Also, take a look at the type of food you are feeding. I used to feed Purina One, and after some good advice and research, I switched to a human grade food (Canidae) and she put on some weight and filled out a little more. They digest and use more of the higher grade food, and at the same time, you don't have to feed as much. I now feed Solid Gold, and she looks perfect.