Iblees was definitely not one of the angels. This is indicated by three things: the clear texts of the Qur'aan, the physical attributes of Iblees and the attitude of Iblees.
1 - With regard to the clear statements of the Qur'aan, Allaah says (interpretation of the meaning):
"And (remember) when We said to the angels: 'Prostrate yourselves unto Adam.' So they prostrated themselves except Iblees (Satan). He was one of the jinn"
Al-Hasan al-Basri said: Iblees was never one of the angels, not even for an instant. He is the father of the jinn, just as Adam (peace be upon him) is the father of mankind.
This was narrated by al-Tabari with a saheeh isnaad as stated by Ibn Katheer in his Tafseer, 3/89.
2 - With regard to his physical attributes, Allaah stated that He created him from fire. Allaah says (interpretation of the meaning):
"He created man (Adam) from sounding clay like the clay of pottery.
And the jinn He created from a smokeless flame of fire"
[al-Rahmaan 55:14, 15]
and it was narrated in Saheeh Muslim (2996) from the hadeeth of 'Aa'ishah (may Allaah be pleased with her) that the Prophet (peace and blessings of Allaah be upon him) said: "The angels were created from light, the jinn were created from smokeless fire, and Adam was created from that which has been described to you."
So it is clear that there is a difference between the physical nature of the angels and that of Iblees, and he is definitely not one of them.
3 - With regard to his attitude, Iblees disobeyed Allaah by not prostrating to Adam, but we know from the Qur'aan that it is impossible for the angels to disobey Allaah. Allaah says (interpretation of the meaning):
"...angels stern (and) severe, who disobey not, (from executing) the Commands they receive from Allaah, but do that which they are commanded"