<div id="myModal" class="modal fade" role="dialog">
<div id="modalContent" class="modal-dialog modal-lg">
    <div class="modal-content">
    // more html


        <div class"buttons">
            <input type="submit" id="printButton" class="btn-lg" value="Print">
            <input type="submit" id="closeButton" class="btn-lg" data-dismiss="modal" value="Close">
        </div>
    </div>

    <script>
        $('#myModal').on('click', 'input#printButton', function () {

            var myCopy = document.body.innerHTML;
            var printThis = document.getElementById("modalContent").innerHTML;
            document.body.innerHTML = printThis;

            window.print();

            document.body.innerHTML = myCopy;

        });
    </script>
</div>