Edit in JSFiddle

<!-- Navigation -->
<nav class="navbar navbar-expand-lg navbar-light bg-light static-top mb-5 shadow">
  <div class="container">
    <a class="navbar-brand" href="#">Start Bootstrap</a>
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarResponsive" aria-controls="navbarResponsive" aria-expanded="false" aria-label="Toggle navigation">
          <span class="navbar-toggler-icon"></span>
        </button>
    <div class="collapse navbar-collapse" id="navbarResponsive">
      <ul class="navbar-nav ml-auto">
        <li class="nav-item active">
          <a class="nav-link" href="#">Home
                <span class="sr-only">(current)</span>
              </a>
        </li>
        <li class="nav-item">
          <a class="nav-link" href="#">About</a>
        </li>
        <li class="nav-item">
          <a class="nav-link" href="#">Services</a>
        </li>
        <li class="nav-item">
          <a class="nav-link" href="#">Contact</a>
        </li>
      </ul>
    </div>
  </div>
</nav>

<!-- Page Content -->
<div class="container">
  <div class="card border-0 shadow my-5">
    <div class="card-body p-5">
      <h1 class="font-weight-light">Fixed Full Page Background Image</h1>
      <p class="lead">In this snippet, the background image is fixed to the body element. Content on the page will scroll, but the image will remain in a fixed position!</p>
      <p class="lead">Scroll down...</p>
      <div style="height: 700px"></div>
      <p class="lead mb-0">You've reached the end!</p>
    </div>
  </div>
</div>
/* Set a background image by replacing the URL below */
body {
  background: url('https://source.unsplash.com/twukN12EN7c/1920x1080') no-repeat center center fixed;
  -webkit-background-size: cover;
  -moz-background-size: cover;
  background-size: cover;
  -o-background-size: cover;
}

              

External resources loaded into this fiddle: