Its funny to find out they classified as progressive...
Yes. Actually, to be honest, Iron Maiden isn’t a real or an actual progressive Metal band. They’re just traditional heavy metal. But a lot of their songs have been considered like other genres, like power metal, speed metal, progressive metal etc. But I can’t deny the fact that they aren’t like progressive metal. They have some long songs with long solos. For example songs like Rime of the Ancient Mariner, Hallowed Be Thy Name, Dance of the Death, Seventh Son of a Seventh Son etc.
Many think calling them progressive metal/rock would be wrong, because progressive rock/metal are more arranged than their songs. But still they totally can’t be ignored is because of their twin lead guitar solos. Which made some songs ''progressed'' and sound like progressive metal.
They were influenced by a variety of 70’s progressive rock bands such as Genesis, Wishbone Ash, Jethro Tull, Rush, and UFO. However, it must be understood that the
New Wave of British Heavy Metal was strongly influenced by progressive rock. This comes with the heavy metal scene’s attempts at distancing themselves from the blues and psychedelic rock roots of the first wave such as Budgie, Black Sabbath, and UFO as they sought to create a more diverse sound in the genre.
This lead them to look at punk, art, and progressive rock for inspiration as they recognized the limitations that came with playing the common blues and psych-based chords with slow 4/4-based tempos. Iron Maiden emerged from this backdrop as they were clearly influenced by Led Zeppelin, Wishbone Ash, and Deep Purple as they preferred guitarists like Richie Blackmore, Johnny Winter, and Pat Travers over Tommy Iommi.
Bruce Dickenson himself was influenced by Peter Hamill, Ian Anderson, and Ian Gillian, all of which were progressive rock singers. So while their sound is clearly rooted in progressive rock, they are not progressive rock as the heavy metal sound they played a role in developing became a genre of metal in itself.
This didn’t change despite adopting progressive rock elements in their later career though. They can be called as progressive, but only in terms of roots and origins instead of being in the genre
per se.