Although the iPhone XI probably won't be announced until September, Apple could begin trial production of it soon, in order to prevent delays.
DigiTimes reports that trial production could begin in the second quarter of 2018 (so any time from April to June), citing 'sources from Taiwan-based IC design houses'.
That's apparently in order to push ahead delivery schedules and ensure there's a high enough yield rate of 3D sensor modules.
No more delays
The iPhone X didn't arrive in stores until a while after it was announced, with rumors of production delays for 3D sensors and other components causing the wait, so it would make sense for Apple to get a head start this year so that the iPhone XI can land in September, shortly after it's announced.
The sources further suggest that mass production of the new iPhone range might begin early in the third quarter of 2018, which should give Apple plenty of time to build up a supply of units before September.
Of course, this is all just rumors for now, and DigiTimes has a mixed track record, but we'd imagine Apple is doing everything it can to avoid another delay.