<div class="carousel-inner" role="listbox">

          <?php if ( have_posts() ) : while ( have_posts() ) : the_post(); $i++; ?>

            <?php if ($i==1) { ?>
                <div class="carousel-item active">
            <?php } else { ?>
                <div class="carousel-item">
            <?php } ?>

                <?php if ( has_post_thumbnail() ) {
                    $url = wp_get_attachment_url( get_post_thumbnail_id() );
                ?>
                <div style="background-image: url('<?php echo $url; ?>')"></div>
                <?php } ?>

            </div><!-- carousel-item active -->