My way of looking at it, I-40 doesn't go to Dallas. It goes to Oklahoma City, at best a three hour drive from the Dallas-Fort Worth area.
I-10 doesn't go to Dallas either, but at least in succession you can take I-10 to I-20 to skim south of Dallas, or I-10 to I-20 to I-30 to go through the heart of Dallas.
If you want to take I-40 for the purpose of visiting some of the interesting places along the way, I suggest taking I-40 east to Amarillo, then US-287 into Fort Worth. But that dumps you into heavy suburban traffic about 30-40 miles outside Fort Worth, maybe 60 miles outside whatever your Dallas area destination might be.
Traffic is so messy around the Dallas-Fort Worth metroplex that I try to approach my actual destination from outside. There are lots of approaches to the DFW area, you can circle around and then drive into your suburban destination, if your destination is one of the suburbs. For example, if destination is Arlington, my route is I-20 across the south fringe, then maybe 360 north. If Denton, I'll go to Oklahoma City then come down I-35. If McKinney or Plano, I would take US-82 across to Sherman and come down US-75, rather than go into the middle of Dallas and come back out.