I have finally managed to get it work now. It was in an iFrame. Here is the code i used.
IWebElement iframe = driver.FindElement(By.Id("ctl00_uxContentPlaceHolderContent_uxQuestionHTMLTextEditor_uxQuestionHTMLTextEditor_editor_DesignIFrame"));
driver.SwitchTo().Frame(iframe);
driver.FindElement(By.XPath("//body[@class='dxheDesignViewArea dxheViewArea']")).SendKeys("Who invented the first fixed wing aircraft?");
driver.SwitchTo().DefaultContent();