Use featured image as background image in WordPress

wordpress-featured-img-bg.php

<?php $thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'full' );?>

<div class="featured-image" style="background-image: url('<?php echo $thumb['0'];?>')">
	
</div>

<style>
.featured-image {
  width:100%;
  height:400px;
  background-size:cover;
}
</style>