Transfer the beans to a large bowl. Add cool water to cover the beans by 3 inches. Let stand for at least 10 hours.
Drain the soaked beans in a colander. Sprinkle the onion and bacon into a large, lined slow cooker. Add the beans, chicken broth and 1 cup cold water. Cover and cook on high until the beans are tender, about 3 hours.
Pour in the tomatoes. Cover and cook on high for 1 1/2 hours. Season with salt and pepper.