Yes, but only if the aircraft is not pressurized.
A really daring pilot can copy the photo above, but only with enough preparation and training can this be done safely. It’s easier to do this with Photoshop.
In fact, many planes even have instructions on how to open the emergency windows as shown in the image below, these instructions are for passengers, but the same mechanism is available for the pilot and co-pilot. The only thing that prevents the opening is the pressure, when the aircraft is pressurized since in most planes the opening happens towards the inside of the aircraft, in cases where the window opens towards the outside there is a safety lock.
So it is possible to remove the window during a flight, but you have to overcome the pressure difference. At lower altitudes this is possible and is a safety feature in situations that obstruct forward vision. If the glass is damaged due to bird strikes or volcanic ash, it is advisable to remove the window before landing.