WASHINGTON — President Trump issued a new specification late Friday for the "big beautiful wall" he's dreamed of building along the U.S.-Mexico border. Now he wants it to be see-through.

And he plans to pick the design personally. That's a common prerogative for billionaire developers. Presidents, however, don't typically interfere in government contract competitions, and it's not clear if he has that authority.

"It has to be a see-through wall," he said at a campaign rally in Huntsville, Ala., for a U.S. senate candidate, adding, "If you can't have vision through it, you don't know who's on the other side."

Whatever it looks like, Trump conceded Friday night that he won't demand construction along every mile of the 2,000-mile frontier, despite the impression he left during the campaign with supporters and wall skeptics alike.

"We are going to have as much wall as we need," he said. "You don't need it all the way. ... You have a lot of natural barriers, et cetera."

The Homeland Security Department's call for prototypes last spring specified concrete as a central element for a barrier envisioned as imposing, hard to climb or pass drugs through or over, and vandal resistant.

Of the 700 miles of barrier already in place along the border, some, but not all, is fencing that allows people on either side to see each across.

Trump's rhetoric in describing the project has at times been more imaginative than the actual specs issued by the Homeland Security Department. Earlier this year he floated the idea of a "solar wall, which would actually look good" and which could even generate enough electricity, he asserted at one point, to pay for itself — a claim belied the economics of photovoltaic technology.

Actual specifications

The idea of a transparent barrier doesn't exactly fit with the specs issued by the Homeland Security Department, which set aside $20 million for prototypes: a "physically imposing" wall 18 to 30 feet, able to withstand tunneling six feet deep, impervious for at least a half-hour to attack by sledgehammer, pickax, blowtorch or other tools, and, from the U.S. side, "aesthetically pleasing."

Department officials have said the barriers could use materials other than concrete for a "see-through component." Border Patrol agents and other security experts have emphasized the need to allow guards on the U.S. side to see what's coming at them — whether a line of trucks, a group of migrants or, smugglers intent on passing illegal drugs to compatriots on the other side.

As Trump put it Friday night in Alabama, a concrete wall isn't good enough because drug smugglers can lob parcels over without U.S. authorities realizing they were in the area.

That's the advice he's gotten from officers in the Border Patrol and Immigration and Customs Enforcement, he said — real experts who know far more than consultants from "the Harvard school of something."

"The take drugs, literally and they throw it — 100 pounds of drugs — they throw it over the wall. They have catapults. They throw it over the wall and it lands, and it hits somebody on the head and you don't even know they are there," Trump said. "Believe it or not, this is the kind of stuff that happens. So you need to have a great wall but it has to be see-through."

Apart from that, he said, "A see-through wall would look better."

In this July 14, 2013, photo, pastors and others raise their arms on the San Diego side of a border fence during a cross-border Sunday religious service with others on the Tijuana, Mexico, side of the fence. (File Photo / The Associated Press)

Trump went out of his way to say there's no need to build along the entire frontier. He's said as much before, but mostly in passing.

"Somebody said what are you going to do? Are you going to build the wall in the middle of the river that nobody can go in?" Trump said Friday night. "Are you going to build that wall on the mountain? You have a mountain which is a wall."

Top advisers, including White House chief of staff John Kelly during his brief stint as homeland security secretary, have said as much. Trump himself has not emphasized such considerations so the shift in tone was noteworthy.

Wall skeptics, including most federal lawmakers from Texas in both parties, have long argued that a barrier anywhere near as extensive as Trump seemed to suggest during the campaign would be wasteful. Much of the border is too remote to attract smugglers or immigrants. And natural features — such as the 1,500-foot deep Santa Elena Canyon in Big Bend — make construction both impossible and unnecessary.

More pragmatic approach

Ongoing resistance in Congress may have nudged Trump to a more pragmatic approach.

In other ways, his latest comments added complications to his dream — for instance, the vow to personally select a winning proposal from among the prototypes being built in San Diego.

"We are looking at four different samples built by four great companies, four different concepts. They're just about completed," Trump said. "I'm going to go out and look at them personally and pick the right one."

Trump made the remarks at an Alabama rally for Sen. Luther Strange, who is in a runoff for the seat to which he was appointed temporarily after Trump tapped Jeff Sessions as attorney general.

He made no mention of trying to get Mexico to pay for the wall. Trump has all but abandoned that central campaign promise.

With Mexico flatly refusing to comply, Trump recently threatened a federal government shutdown if Democrats in Congress block a $1.6 billion initial outlay that would cover 60 miles of new barrier in Texas, and with 14 miles of renovated fencing in Southern California.

"The wall is happening folks, believe me," he said in Alabama. "The wall is happening."