While in case most VTOL aircraft, the rotors are tilted, one unique case is the Curtis Wright X-19, which had what the company called tilt-propellers. Curtis Wright at that point was a propeller manufacturer and its designers made the aircraft around the propeller.
Image from warbirdinformationexchange.org
This in turn, was developed from the X-100
Image from airandspace.si.edu
The designers used a property of the propeller called the radial lift force:
... as a propeller is inclined towards the vertical from the horizontal, the resultant of the propeller's thrust and the pressure of the relative wind acting on the rotating disk is a force with an additional lift component in the vertical axis. ... short propellers with wide blades magnified the radial force effect by increasing the surface area of the propeller disk without the compressibility issues of longer rotor blades. This offered a potential advantage over other tilt-rotor models, such as the Bell XV-3, with longer and narrower blades that did not have sufficient surface area to take advantage of the phenomenon. The added lift generated from the radial force permitted an aircraft built with the specially constructed lifting propellers to have smaller wings, which decreased weight and high-speed drag. This resulted in the most aerodynamically efficient of all of the VTOL designs.
In hover, the roll control was provided by differential pitch of the propellers. The aircraft basically behaved like a high-disc loading helicopter and was statically unstable in hover and in pitch and roll at low speed. The aircraft crashed during its first transition flight in 1965, with the progra ending soon.
This method is also used in a number of small UAVs.