Initially, atmospheric craft were only built for training, such as the X-1 and X-2 solid fuel rockets.
The X-1 and X-2 both trained crew for the first Discovery launches.
The X-2 is still used for launch training, providing some of the the same forces as an actual orbital launch and allowing Kerbals to also experience re-entry.
A slightly improved model known as the X-3 is in use as well, and a still-under-development replacement, tenatively called the X-3A, is scheduled to replace it.
The first true aircraft was the X-4 Jetplane. This small one Kerbal craft was designed in cooperation with the KSGS, the Kerbal States Geological Survey to explore anomalies and other features of Kerbin.
The X-4 was initially very successfull, investigating the airfield off the coast from the Space Center.
Unfortunately, the X-4 failed catastrophically during a mission to map one of the Monolith Anomalies, in the rough mountainous terrain west of the Space Center.
Thankfully Jeb was able to walk away from the crash, but he later reported that it was a “close one.”
Development of a new craft capable of scouting Kerbin was actively pursued following the X-4 crash, a bigger, better and safer plane, the X-5.
On Year Two, Day 372 KSEA released plans for the X-5!
On Year Two, Day 358 Mitster took it on its first flight and reported it was responsive and very fast! The new J-404 engines prove to be decent for fuel economy.
With several successful flights (and landings!) the X-5 has proven itself to be a capable craft that can easily reach most destinations on Kerbin!
KSEA commissions construction of an additional X-5, and pilots begin debating what to name the new vehicles!