Why doesn't justify-content: stretch work?

Why does the code below not stretch my 100px items?

The results are basically close to this:

[100px|100px|100px|.........]

That looks a lot like flex-start, not flex-stretch. Is this the intended behavior?

I'm looking for:

[...100px...100px...100px...]

.box {  display: flex;  justify-content: stretch;  width: 500px;}.item {  width: 100px;}

<div class="box"><div class="item">1</div><div class="item">2</div><div class="item">3</div></div>