Though a bench press is mostly chest, your biceps are needed to bend and straighten your elbow. Here's how to a dumbbell bench press:
Lay on a bench or on the floor with a dumbbell in each hand, elbows bent, and your arms at a 45 degree angle. If on the floor bend your knees and put your feet flat on the floor.
Push the dumbbells above your head until your arms are straight
Bend your elbows to return to the starting position.