Lockheed J37
...that the USA's first locally designed jet engine, the Lockheed J37, spent ten years in development but was never used on a production aircraft?
...that the <a class="mw-redirect" href="./USA" id="mwEeU" rel="mw:WikiLink" title="USA">USA</a>'s first locally designed <a href="./Jet_engine" id="mwEeY" rel="mw:WikiLink" title="Jet engine">jet engine</a>, the <b id="mwEec"><a href="./Lockheed_J37" id="mwEeg" rel="mw:WikiLink" title="Lockheed J37">Lockheed J37</a></b>, spent ten years in development but was never used on a production aircraft?