#container {

 width: 800px;
  display: flex;
  flex-wrap: wrap;
}

.cell {
  width: 300px;
  flex; 1 auto;
}


<div id="container">

<div class="cell">
Cells with arbitrarily long content.</div>

<div class="cell">
</div>

<div class="cell">
</div>

<div class="cell">
</div>

<div class="cell">
</div>

</div>

</body>
</html>

#container {

 width: 800px;
  display: flex;
  flex-wrap: wrap;
  align-items: flex-start;
}

.cell {
  width: 300px;
  flex: 1 auto;
  padding: 10px;
  border: 1px solid red;
}