Is it possible to fly into space in an airplane?

The fundamental difference between an airplane and a rocket is the engine. The jet engine (air-jet engine of an airplane), in airless space (or in very rarefied layers) will suffocate, because it needs to consume oxygen, for the fuel combustion process.

In a rocket engine, there are fueled chemical mixtures that cause the fuel to burn, so it does not need to draw oxygen from the atmosphere.

Otherwise, it’s not about first space speed, it’s about the fact that in order for an airplane to fly into space, it must become a rocket, or its engines will stop working.