All perfect praise be to Allah, The Lord of The Worlds; and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Ihram (State of ritual consecration) for the person offering Hajj or Umrah should be assumed from the Miqat (Place where one should assume the state of (Ihram) that he passes by or the closest one parallel to him, whether he was travelling by plane or by land. The Miqat for the person coming from Riyad is Qarn-Ul-Manazil, which is called today [As-Sail Al-Kabeer] and it is impermissible to assume Ihram from Jeddah because it is inside the Miqat. Ibn Abbas (May Allah be pleased with them) narrated, "Allah's Messenger (PBUH) had fixed Dhul Hulaifa as the Miqat for the people of Medina; Al-Juhfa for the people of Sham; and Qarn Ul-Manazil for the people of Najd; and Yalamlam for the people of Yemen.
So, these (above mentioned) are the Mawaqit for all those living at those places, and besides them for those who come through those places with the intention of performing Hajj and `Umra and whoever lives within these places should assume Ihram from his dwelling place, and similarly the people of Mecca can assume lhram from Mecca." {Agreed upon}.
It is permissible for you to prepare for Ihram by bathing while at your dwelling place, then put on the Ihram wear there, or at the airport, or aboard the plane before reaching the Meeqat. Usually, the captain alerts the passengers about the time of reaching the Meeqat, and when he does so you should make the intention of offering Umrah by heart and it is recommended that you say it.
However, there is no harm if you made the intention of Ihram before reaching the Miqat in order to be on the safe side, although it is better to assume it from the Miqat. And Allah knows best.