form onsubmit ignores action

I am new to JavaScript. I have a form with an action and I need to fill the form inputs using JavaScript on submitting. I use an onsubmit function which executes as expected, but the form didn't hit the action in the action attribute and no data is sent to the server.
Can anybody help me find what is missing ?

JavaScript/jQuery:

$("#myForm").submit(function (e) {
  e.preventDefault();
  return fillFormValues();
})

function fillFormValues()
{
  $.get( ...,function( data ) {
    //fill my inputs values here
    return true;
  });
  return false;
}

HTML:

<form action="..." method="post" target="output_frame" id="myForm">
    <input name="first" id="first" type="hidden" value="">
    ...
    <input type="submit"  value="next">
</form>